Add output abstraction and reorganize profile

* Moves ColorSystem from Profile to Capabilities
* Renames Tty to IsTerminal
* Adds IAnsiConsoleOutput to make output more flexible

Closes #343
Closes #359
Closes #369

src/Spectre.Console/AnsiConsoleSettings.cs

@@ -1,5 +1,4 @@
 using System.Collections.Generic;
-using System.IO;
 
 namespace Spectre.Console
 {
@@ -22,7 +21,7 @@ namespace Spectre.Console
         /// <summary>
         /// Gets or sets the out buffer.
         /// </summary>
-        public TextWriter? Out { get; set; }
+        public IAnsiConsoleOutput? Out { get; set; }
 
         /// <summary>
         /// Gets or sets a value indicating whether or not the