Add enhancements to progress widget

* Adds TransferSpeedColumn
* Adds DownloadedColumn
* Adds ElapsedTimeColumn
* Minor enhancements to existing columns

src/Spectre.Console/Widgets/Progress/Columns/DownloadedColumn.cs

@@ -0,0 +1,42 @@
+using System;
+using System.Globalization;
+using Spectre.Console.Internal;
+using Spectre.Console.Rendering;
+
+namespace Spectre.Console
+{
+    /// <summary>
+    /// A column showing download progress.
+    /// </summary>
+    public sealed class DownloadedColumn : ProgressColumn
+    {
+        /// <summary>
+        /// Gets or sets the <see cref="CultureInfo"/> to use.
+        /// </summary>
+        public CultureInfo? Culture { get; set; }
+
+        /// <inheritdoc/>
+        public override IRenderable Render(RenderContext context, ProgressTask task, TimeSpan deltaTime)
+        {
+            var total = new FileSize(task.MaxValue);
+
+            if (task.IsFinished)
+            {
+                return new Markup(string.Format(
+                    "[green]{0} {1}[/]",
+                    total.Format(Culture),
+                    total.Suffix));
+            }
+            else
+            {
+                var downloaded = new FileSize(task.Value, total.Unit);
+
+                return new Markup(string.Format(
+                    "{0}[grey]/[/]{1} [grey]{2}[/]",
+                    downloaded.Format(Culture),
+                    total.Format(Culture),
+                    total.Suffix));
+            }
+        }
+    }
+}

src/Spectre.Console/Widgets/Progress/Columns/ElapsedTimeColumn.cs

@@ -0,0 +1,37 @@
+using System;
+using Spectre.Console.Rendering;
+
+namespace Spectre.Console
+{
+    /// <summary>
+    /// A column showing the elapsed time of a task.
+    /// </summary>
+    public sealed class ElapsedTimeColumn : ProgressColumn
+    {
+        /// <inheritdoc/>
+        protected internal override bool NoWrap => true;
+
+        /// <summary>
+        /// Gets or sets the style of the remaining time text.
+        /// </summary>
+        public Style Style { get; set; } = new Style(foreground: Color.Blue);
+
+        /// <inheritdoc/>
+        public override IRenderable Render(RenderContext context, ProgressTask task, TimeSpan deltaTime)
+        {
+            var elapsed = task.ElapsedTime;
+            if (elapsed == null)
+            {
+                return new Markup("-:--:--");
+            }
+
+            return new Text($"{elapsed.Value:h\\:mm\\:ss}", Style ?? Style.Plain);
+        }
+
+        /// <inheritdoc/>
+        public override int? GetColumnWidth(RenderContext context)
+        {
+            return 7;
+        }
+    }
+}

src/Spectre.Console/Widgets/Progress/Columns/SpinnerColumn.cs

@@ -16,6 +16,7 @@ namespace Spectre.Console
         private readonly object _lock;
         private Spinner _spinner;
         private int? _maxWidth;
+        private string? _completed;
 
         /// <inheritdoc/>
         protected internal override bool NoWrap => true;
@@ -36,6 +37,25 @@ namespace Spectre.Console
             }
         }
 
+        /// <summary>
+        /// Gets or sets the text that should be shown instead
+        /// of the spinner once a task completes.
+        /// </summary>
+        public string? CompletedText
+        {
+            get => _completed;
+            set
+            {
+                _completed = value;
+                _maxWidth = null;
+            }
+        }
+
+        /// <summary>
+        /// Gets or sets the completed style.
+        /// </summary>
+        public Style? CompletedStyle { get; set; }
+
         /// <summary>
         /// Gets or sets the style of the spinner.
         /// </summary>
@@ -67,7 +87,7 @@ namespace Spectre.Console
 
             if (!task.IsStarted || task.IsFinished)
             {
-                return new Markup(new string(' ', GetMaxWidth(context)));
+                return new Markup(CompletedText ?? " ", CompletedStyle ?? Style.Plain);
             }
 
             var accumulated = task.State.Update<double>(ACCUMULATED, acc => acc + deltaTime.TotalMilliseconds);
@@ -97,7 +117,9 @@ namespace Spectre.Console
                     var useAscii = (context.LegacyConsole || !context.Unicode) && _spinner.IsUnicode;
                     var spinner = useAscii ? Spinner.Known.Ascii : _spinner ?? Spinner.Known.Default;
 
-                    _maxWidth = spinner.Frames.Max(frame => Cell.GetCellLength(context, frame));
+                    _maxWidth = Math.Max(
+                        ((IRenderable)new Markup(CompletedText ?? " ")).Measure(context, int.MaxValue).Max,
+                        spinner.Frames.Max(frame => Cell.GetCellLength(context, frame)));
                 }
 
                 return _maxWidth.Value;

src/Spectre.Console/Widgets/Progress/Columns/TransferSpeedColumn.cs

@@ -0,0 +1,30 @@
+using System;
+using System.Globalization;
+using Spectre.Console.Internal;
+using Spectre.Console.Rendering;
+
+namespace Spectre.Console
+{
+    /// <summary>
+    /// A column showing transfer speed.
+    /// </summary>
+    public sealed class TransferSpeedColumn : ProgressColumn
+    {
+        /// <summary>
+        /// Gets or sets the <see cref="CultureInfo"/> to use.
+        /// </summary>
+        public CultureInfo? Culture { get; set; }
+
+        /// <inheritdoc/>
+        public override IRenderable Render(RenderContext context, ProgressTask task, TimeSpan deltaTime)
+        {
+            if (task.Speed == null)
+            {
+                return new Text("?/s");
+            }
+
+            var size = new FileSize(task.Speed.Value);
+            return new Markup(string.Format("{0}/s", size.ToString(suffix: true, Culture)));
+        }
+    }
+}

src/Spectre.Console/Widgets/Progress/ProgressTask.cs

@@ -89,7 +89,14 @@ namespace Spectre.Console
         /// </summary>
         public TimeSpan? RemainingTime => GetRemainingTime();
 
-        internal ProgressTask(int id, string description, double maxValue, bool autoStart)
+        /// <summary>
+        /// Initializes a new instance of the <see cref="ProgressTask"/> class.
+        /// </summary>
+        /// <param name="id">The task ID.</param>
+        /// <param name="description">The task description.</param>
+        /// <param name="maxValue">The task max value.</param>
+        /// <param name="autoStart">Whether or not the task should start automatically.</param>
+        public ProgressTask(int id, string description, double maxValue, bool autoStart = true)
         {
             _samples = new List<ProgressSample>();
             _lock = new object();