Propagate a did-use-grammar event out of the tokenized buffer.

src/text-editor.coffee

@@ -325,6 +325,9 @@ class TextEditor extends Model
   onDidChangeGrammar: (callback) ->
     @emitter.on 'did-change-grammar', callback
 
+  onDidUseGrammar: (callback) ->
+    @displayBuffer.onDidUseGrammar(callback)
+
   # Extended: Calls your `callback` when the result of {::isModified} changes.
   #
   # * `callback` {Function}