Added Codility to interactive tutorials

Author: grogy

free-programming-interactive-tutorials-en.md

@@ -12,6 +12,7 @@
 * [HTML / CSS](#html--css)
 * [Java](#java)
 * [Javascript](#javascript)
+* [Language Agnostic](#language-agnostic)
 * [Lisp](#lisp)
 * [MATLAB](#matlab)
 * [Node](#node)
@@ -94,6 +95,10 @@
 * [Try jQuery](http://try.jquery.com/)
 
 
+### Language Agnostic
+* [Codility](https://codility.com/programmers/)
+
+
 ### Lisp
 * [Lisp Koans](https://github.com/google/lisp-koans)