Added initial support for rendering composites

This is far from complete, but it's a start
and it will enable us to create things like tables
and other complex objects in the long run.

src/Spectre.Console/Internal/Extensions/CharExtensions.cs

@@ -0,0 +1,12 @@
+using System.Text;
+
+namespace Spectre.Console.Internal
+{
+    internal static class CharExtensions
+    {
+        public static int CellLength(this char token, Encoding encoding)
+        {
+            return Cell.GetCellLength(encoding, token);
+        }
+    }
+}

src/Spectre.Console/Internal/Extensions/ConsoleExtensions.cs

@@ -1,10 +1,25 @@
 using System;
 using System.Diagnostics.CodeAnalysis;
+using Spectre.Console.Composition;
 
 namespace Spectre.Console.Internal
 {
     internal static class ConsoleExtensions
     {
+        public static IDisposable PushAppearance(this IAnsiConsole console, Appearance appearance)
+        {
+            if (console is null)
+            {
+                throw new ArgumentNullException(nameof(console));
+            }
+
+            var current = new Appearance(console.Foreground, console.Background, console.Style);
+            console.SetColor(appearance.Foreground, true);
+            console.SetColor(appearance.Background, false);
+            console.Style = appearance.Style;
+            return new AppearanceScope(console, current);
+        }
+
         public static IDisposable PushColor(this IAnsiConsole console, Color color, bool foreground)
         {
             if (console is null)
@@ -47,6 +62,33 @@ namespace Spectre.Console.Internal
         }
     }
 
+    internal sealed class AppearanceScope : IDisposable
+    {
+        private readonly IAnsiConsole _console;
+        private readonly Appearance _apperance;
+
+        public AppearanceScope(IAnsiConsole console, Appearance appearance)
+        {
+            _console = console ?? throw new ArgumentNullException(nameof(console));
+            _apperance = appearance;
+        }
+
+        [SuppressMessage("Design", "CA1065:Do not raise exceptions in unexpected locations")]
+        [SuppressMessage("Performance", "CA1821:Remove empty Finalizers")]
+        ~AppearanceScope()
+        {
+            throw new InvalidOperationException("Appearance scope was not disposed.");
+        }
+
+        public void Dispose()
+        {
+            GC.SuppressFinalize(this);
+            _console.SetColor(_apperance.Foreground, true);
+            _console.SetColor(_apperance.Background, false);
+            _console.Style = _apperance.Style;
+        }
+    }
+
     internal sealed class ColorScope : IDisposable
     {
         private readonly IAnsiConsole _console;

src/Spectre.Console/Internal/Extensions/StringExtensions.cs

@@ -0,0 +1,36 @@
+using System;
+using System.Text;
+
+namespace Spectre.Console.Internal
+{
+    internal static class StringExtensions
+    {
+        // Cache whether or not internally normalized line endings
+        // already are normalized. No reason to do yet another replace if it is.
+        private static readonly bool _alreadyNormalized
+            = Environment.NewLine.Equals("\n", StringComparison.OrdinalIgnoreCase);
+
+        public static int CellLength(this string text, Encoding encoding)
+        {
+            return Cell.GetCellLength(encoding, text);
+        }
+
+        public static string NormalizeLineEndings(this string text, bool native = false)
+        {
+            var normalized = text?.Replace("\r\n", "\n")
+                ?.Replace("\r", string.Empty);
+
+            if (native && !_alreadyNormalized)
+            {
+                normalized = normalized.Replace("\n", Environment.NewLine);
+            }
+
+            return normalized;
+        }
+
+        public static string[] SplitLines(this string text)
+        {
+            return text.NormalizeLineEndings().Split(new[] { '\n' }, StringSplitOptions.None);
+        }
+    }
+}