Add support for indeterminate progress

This commit also changes the behavior of ProgressContext.IsFinished.
Only tasks that have been started will be taken into consideration,
and not indeterminate tasks.

Closes #329
Closes #331

src/Spectre.Console/Extensions/EnumerableExtensions.cs

@@ -6,6 +6,17 @@ namespace Spectre.Console
 {
     internal static class EnumerableExtensions
     {
+        public static IEnumerable<T> Repeat<T>(this IEnumerable<T> source, int count)
+        {
+            while (count-- > 0)
+            {
+                foreach (var item in source)
+                {
+                    yield return item;
+                }
+            }
+        }
+
         public static int IndexOf<T>(this IEnumerable<T> source, T item)
             where T : class
         {

src/Spectre.Console/Extensions/Progress/ProgressTaskExtensions.cs

@@ -57,5 +57,22 @@ namespace Spectre.Console
             task.Value = value;
             return task;
         }
+
+        /// <summary>
+        /// Sets whether the task is considered indeterminate or not.
+        /// </summary>
+        /// <param name="task">The task.</param>
+        /// <param name="indeterminate">Whether the task is considered indeterminate or not.</param>
+        /// <returns>The same instance so that multiple calls can be chained.</returns>
+        public static ProgressTask IsIndeterminate(this ProgressTask task, bool indeterminate = true)
+        {
+            if (task is null)
+            {
+                throw new ArgumentNullException(nameof(task));
+            }
+
+            task.IsIndeterminate = indeterminate;
+            return task;
+        }
     }
 }

src/Spectre.Console/Extensions/RenderableExtensions.cs

@@ -27,7 +27,7 @@ namespace Spectre.Console
                 throw new ArgumentNullException(nameof(renderable));
             }
 
-            var context = new RenderContext(console.Profile.Capabilities);
+            var context = new RenderContext(console.Profile.ColorSystem, console.Profile.Capabilities);
             var renderables = console.Pipeline.Process(context, new[] { renderable });
 
             return GetSegments(console, context, renderables);