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/runtime.js

@@ -90,7 +90,9 @@ export function template(templateSpec, env) {
     invokePartial: invokePartialWrapper,
 
     fn: function(i) {
-      return templateSpec[i];
+      let ret = templateSpec[i];
+      ret.decorator = templateSpec[i + '_d'];
+      return ret;
     },
 
     programs: [],
@@ -142,7 +144,17 @@ export function template(templateSpec, env) {
       }
     }
 
-    return '' + templateSpec.main(container, context, container.helpers, container.partials, data, blockParams, depths);
+    function main(context/*, options*/) {
+      return '' + templateSpec.main(container, context, container.helpers, container.partials, data, blockParams, depths);
+    }
+
+    if (templateSpec.main_d) {
+      // Note that we are ignoring the props value here as we apply things slightly differently
+      // when applying decorators to the root function.
+      main = templateSpec.main_d(main, {}, container, undefined, data, blockParams, depths);
+    }
+
+    return main(context, options);
   }
   ret.isTop = true;
 
@@ -190,6 +202,13 @@ export function wrapProgram(container, i, fn, data, declaredBlockParams, blockPa
         blockParams && [options.blockParams].concat(blockParams),
         currentDepths);
   }
+
+  if (fn.decorator) {
+    let props = {};
+    prog = fn.decorator(prog, props, container, depths && depths[0], data, blockParams, depths);
+    Utils.extend(prog, props);
+  }
+
   prog.program = i;
   prog.depth = depths ? depths.length : 0;
   prog.blockParams = declaredBlockParams || 0;