Add context code support to exercises (optional)

Partial #52, fixes #49, fixes #46

theme/javascript/execute/javascript.js

@@ -2,6 +2,7 @@ define(function() {
     return {
         id: "javascript",
         assertCode: "function assert(condition, message) { \nif (!condition) { \n throw message || \"Assertion failed\"; \n } \n }\n",
-        REPL: JSREPL
+        REPL: JSREPL,
+        sep: ";\n",
     };
-});
+});