Re-render markdown when a grammar is added

Previously if the markdown preview launched before grammars finished
loading the fenced code blocks would not be colorized correctly.

src/packages/markdown-preview/lib/markdown-preview-view.coffee

@@ -37,6 +37,7 @@ class MarkdownPreviewView extends ScrollView
     super
 
     @renderMarkdown()
+    syntax.on 'grammar-added', => @renderMarkdown()
     @on 'core:move-up', => @scrollUp()
     @on 'core:move-down', => @scrollDown()
 
@@ -72,7 +73,6 @@ class MarkdownPreviewView extends ScrollView
   setLoading: ->
     @html($$$ -> @div class: 'markdown-spinner', 'Loading Markdown...')
 
-
   tokenizeCodeBlocks: (html) =>
     html = $(html)
     preList = $(html.filter("pre"))

src/packages/markdown-preview/spec/markdown-preview-spec.coffee

@@ -122,3 +122,13 @@ describe "MarkdownPreview package", ->
               pane1.activeItem.buffer.trigger 'saved'
               expect(preview.renderMarkdown).toHaveBeenCalled()
               expect(pane2.activeItem).toBe preview
+
+      describe "when a new grammar is loaded", ->
+        it "reloads the view to colorize any fenced code blocks matching the newly loaded grammar", ->
+          rootView.getActiveView().trigger 'markdown-preview:show'
+          [pane1, pane2] = rootView.getPanes()
+          preview = pane2.activeItem
+          preview.renderMarkdown.reset()
+
+          atom.activatePackage('javascript-tmbundle', sync: true)
+          expect(preview.renderMarkdown).toHaveBeenCalled()