Implement block decorators

These allow for a given block to be wrapped in helper methods or metadata and allow for more control over the current container and method before the code is run.

lib/handlebars/compiler/code-gen.js

@@ -69,6 +69,9 @@ function CodeGen(srcFile) {
 }
 
 CodeGen.prototype = {
+  isEmpty() {
+    return !this.source.length;
+  },
   prepend: function(source, loc) {
     this.source.unshift(this.wrap(source, loc));
   },