Make YesNtInterpreter usable as a library and add AddStatement method to be able to define custom statement

YesNt.Interpreter.App/Program.cs

@@ -0,0 +1,14 @@
+using System;
+
+using YesNt.Interpreter.Runtime;
+
+if (args.Length == 1)
+{
+    YesNtInterpreter interpreter = new YesNtInterpreter();
+    interpreter.Initialize();
+    interpreter.Execute(args[0]);
+}
+else
+{
+    Console.WriteLine("No path specified!");
+}