Change IAnsiConsole to render IRenderable

This makes it possible for encoders to output better representation
of the actual objects instead of working with chopped up segments.

* IAnsiConsole.Write now takes an IRenderable instead of segments
* Calculating cell width does no longer require a render context
* Removed RenderContext.LegacyConsole
* Removed RenderContext.Encoding
* Added Capabilities.Unicode

src/Spectre.Console/Extensions/RenderableExtensions.cs

@@ -0,0 +1,47 @@
+using System;
+using System.Collections.Generic;
+using Spectre.Console.Rendering;
+
+namespace Spectre.Console
+{
+    /// <summary>
+    /// Contains extension methods for <see cref="IRenderable"/>.
+    /// </summary>
+    public static class RenderableExtensions
+    {
+        /// <summary>
+        /// Gets the segments for a renderable using the specified console.
+        /// </summary>
+        /// <param name="renderable">The renderable.</param>
+        /// <param name="console">The console.</param>
+        /// <returns>An enumerable containing segments representing the specified <see cref="IRenderable"/>.</returns>
+        public static IEnumerable<Segment> GetSegments(this IRenderable renderable, IAnsiConsole console)
+        {
+            if (console is null)
+            {
+                throw new ArgumentNullException(nameof(console));
+            }
+
+            if (renderable is null)
+            {
+                throw new ArgumentNullException(nameof(renderable));
+            }
+
+            var context = new RenderContext(console.Profile.Capabilities);
+            var renderables = console.Pipeline.Process(context, new[] { renderable });
+
+            return GetSegments(console, context, renderables);
+        }
+
+        private static IEnumerable<Segment> GetSegments(IAnsiConsole console, RenderContext options, IEnumerable<IRenderable> renderables)
+        {
+            var result = new List<Segment>();
+            foreach (var renderable in renderables)
+            {
+                result.AddRange(renderable.Render(options, console.Profile.Width));
+            }
+
+            return Segment.Merge(result);
+        }
+    }
+}