Add enhancements to progress widget

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

src/Spectre.Console.Tests/Unit/Progress/DownloadedColumnTests.cs

@@ -0,0 +1,30 @@
+using System.Globalization;
+using Shouldly;
+using Xunit;
+
+namespace Spectre.Console.Tests.Unit
+{
+    public sealed class DownloadedColumnTests
+    {
+        [Theory]
+        [InlineData(0, 1, "0/1 byte")]
+        [InlineData(37, 101, "37/101 bytes")]
+        [InlineData(101, 101, "101 bytes")]
+        [InlineData(512, 1024, "0.5/1.0 KB")]
+        [InlineData(1024, 1024, "1.0 KB")]
+        [InlineData(1024 * 512, 5 * 1024 * 1024, "0.5/5.0 MB")]
+        [InlineData(5 * 1024 * 1024, 5 * 1024 * 1024, "5.0 MB")]
+        public void Should_Return_Correct_Value(double value, double total, string expected)
+        {
+            // Given
+            var fixture = new ProgressColumnFixture<DownloadedColumn>(value, total);
+            fixture.Column.Culture = CultureInfo.InvariantCulture;
+
+            // When
+            var result = fixture.Render();
+
+            // Then
+            result.ShouldBe(expected);
+        }
+    }
+}

src/Spectre.Console.Tests/Unit/Progress/ProgressColumnFixture.cs

@@ -0,0 +1,29 @@
+using System;
+using System.Text;
+using Spectre.Console.Rendering;
+using Spectre.Console.Testing;
+
+namespace Spectre.Console.Tests.Unit
+{
+    public sealed class ProgressColumnFixture<T>
+        where T : ProgressColumn, new()
+    {
+        public T Column { get; }
+        public ProgressTask Task { get; set; }
+
+        public ProgressColumnFixture(double completed, double total)
+        {
+            Column = new T();
+            Task = new ProgressTask(1, "Foo", total);
+            Task.Increment(completed);
+        }
+
+        public string Render()
+        {
+            var console = new FakeConsole();
+            var context = new RenderContext(Encoding.UTF8, false);
+            console.Render(Column.Render(context, Task, TimeSpan.Zero));
+            return console.Output;
+        }
+    }
+}