Add live display support

This commit also adds functionality to LiveRenderable that should
fix some problems related to vertical overflow.

Closes #316
Closes #415

docs/input/live/index.cshtml

@@ -1,4 +1,4 @@
-Title: Live Displays
+Title: Live
 Order: 4
 ---
 

docs/input/live/live-display.md

@@ -0,0 +1,62 @@
+Title: Live Display
+Order: 0
+---
+
+Spectre.Console can update arbitrary widgets in-place. 
+
+<?# Alert ?>
+  The live display is not 
+  thread safe, and using it together with other interactive components such as 
+  prompts, status displays or other progress displays are not supported.
+<?#/ Alert ?>
+
+```csharp
+var table = new Table().Centered();
+
+AnsiConsole.Live(table)
+    .Start(ctx => 
+    {
+        table.AddColumn("Foo");
+        ctx.Refresh();
+        Thread.Sleep(1000);
+
+        table.AddColumn("Bar");
+        ctx.Refresh();
+        Thread.Sleep(1000);
+    });
+```
+
+## Asynchronous progress
+
+If you prefer to use async/await, you can use `StartAsync` instead of `Start`.
+
+```csharp
+var table = new Table().Centered();
+
+await AnsiConsole.Live(table)
+    .StartAsync(async ctx => 
+    {
+        table.AddColumn("Foo");
+        ctx.Refresh();
+        await Task.Delay(1000);
+
+        table.AddColumn("Bar");
+        ctx.Refresh();
+        await Task.Delay(1000);
+    });
+```
+
+## Configure
+
+```csharp
+var table = new Table().Centered();
+
+AnsiConsole.Live(table)
+    .AutoClear(false)   // Do not remove when done
+    .Overflow(VerticalOverflow.Ellipsis) // Show ellipsis when overflowing
+    .Cropping(VerticalOverflowCropping.Top) // Crop overflow at top
+    .Start(ctx =>
+    {
+        // Omitted
+    });
+```

docs/input/live/progress.md

@@ -66,7 +66,6 @@ await AnsiConsole.Progress()
 ## Configure
 
 ```csharp
-// Asynchronous
 AnsiConsole.Progress()
     .AutoRefresh(false) // Turn off auto refresh
     .AutoClear(false)   // Do not remove the task list when done

docs/input/live/status.md

@@ -1,5 +1,5 @@
 Title: Status
-Order: 6
+Order: 10
 RedirectFrom: status
 ---