Add method to write VT/ANSI control codes

Adds a new extension method IAnsiConsole.WriteAnsi that writes
VT/ANSI control codes to the console. If VT/ANSI control codes are
not supported, nothing will be written.

src/Spectre.Console/Capabilities.cs

@@ -16,7 +16,7 @@ namespace Spectre.Console
 
         /// <summary>
         /// Gets or sets a value indicating whether or not
-        /// the console supports Ansi.
+        /// the console supports VT/ANSI control codes.
         /// </summary>
         public bool Ansi { get; set; }