Add link support for supported terminals

Also refactors the code quite a bit, to make it a bit more
easier to add features like this in the future.

Closes #75

src/Spectre.Console/Internal/Constants.cs

@@ -4,5 +4,7 @@ namespace Spectre.Console.Internal
     {
         public const int DefaultBufferWidth = 80;
         public const int DefaultBufferHeight = 9001;
+
+        public const string EmptyLink = "https://emptylink";
     }
 }