Re-tokenize buffer when its grammar is updated

This can happen if a grammar that the grammar includes is added or
removed from the syntax global.

src/app/language-mode.coffee

@@ -34,6 +34,7 @@ class LanguageMode
     return if grammar is @grammar
     @grammar = grammar
     @currentGrammarScore = score ? grammar.getScore(@buffer.getPath(), @buffer.getText())
+    @subscribe @grammar, 'grammar-updated', => @trigger 'grammar-updated'
     @trigger 'grammar-changed', grammar
 
   reloadGrammar: ->

src/app/null-grammar.coffee

@@ -1,4 +1,6 @@
 Token = require 'token'
+EventEmitter = require 'event-emitter'
+_ = require 'underscore'
 
 ###
 # Internal #
@@ -14,3 +16,5 @@ class NullGrammar
     { tokens: [new Token(value: line, scopes: ['null-grammar.text.plain'])] }
 
   grammarAddedOrRemoved: -> # no op
+
+_.extend NullGrammar.prototype, EventEmitter

src/app/tokenized-buffer.coffee

@@ -28,6 +28,7 @@ class TokenizedBuffer
     @resetScreenLines()
     @buffer.on "changed.tokenized-buffer#{@id}", (e) => @handleBufferChange(e)
     @languageMode.on 'grammar-changed', => @resetScreenLines()
+    @languageMode.on 'grammar-updated', => @resetScreenLines()
 
   resetScreenLines: ->
     @screenLines = @buildPlaceholderScreenLinesForRows(0, @buffer.getLastRow())