Changes magic sequence to '!#mojo ' and strips 'mojo ' when building url

examples/recipes/recipe_handler/sample

-#!mojo:recipe_handler
+#!mojo mojo:recipe_handler
 {
   // URL of the app responsible for rendering this recipe.
   "renderer" : "mojo:bar",
 
   // Set of ingredients that comprise this recipe. The recipe connects to each
   // of the apps listed here.
   "ingredients": [
     {
       "url": "mojo:foo",
       // TODO(sky): add some way to constrain set of values an ingredient sees.
     },
   ],
 
   // Set of key/value pairs that are exposed to all ingredients by way of
   // RecipeValueStore.
   "values": {
     "key": "value",
   },
 }