Allow preview of editor if grammar is source.gfm

Previously only markdow file extensions were considered
but any editor whose grammar is GitHub Markdow should have
preview enabled.

Closes #277

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

@@ -26,7 +26,7 @@ class MarkdownPreviewView extends ScrollView
       @attach()
 
   attach: ->
-    return unless @isMarkdownFile(@getActivePath())
+    return unless @isMarkdownEditor()
     rootView.append(this)
     @markdownBody.html(@getLoadingHtml())
     @loadHtml()
@@ -39,9 +39,6 @@ class MarkdownPreviewView extends ScrollView
     rootView.focus()
     @detaching = false
 
-  getActivePath: ->
-    rootView.getActiveEditor()?.getPath()
-
   getActiveText: ->
     rootView.getActiveEditor()?.getText()
 
@@ -76,5 +73,8 @@ class MarkdownPreviewView extends ScrollView
   setHtml: (html) ->
     @markdownBody.html(html) if @hasParent()
 
-  isMarkdownFile: (path) ->
+  isMarkdownEditor: (path) ->
+    editor = rootView.getActiveEditor()
+    return unless editor?
+    return true if editor.getGrammar().scopeName is 'source.gfm'
     path and fs.isMarkdownExtension(fs.extension(path))

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

@@ -1,6 +1,7 @@
 $ = require 'jquery'
 RootView = require 'root-view'
 MarkdownPreview = require 'markdown-preview/lib/markdown-preview-view'
+_ = require 'underscore'
 
 describe "MarkdownPreview", ->
   beforeEach ->
@@ -33,6 +34,18 @@ describe "MarkdownPreview", ->
       markdownPreviewView = rootView.find('.markdown-preview')?.view()
       expect(markdownPreviewView.loadHtml).toHaveBeenCalled()
 
+    it "displays a preview for a file with the source.gfm grammar scope", ->
+      gfmGrammar = _.find syntax.grammars, (grammar) -> grammar.scopeName is 'source.gfm'
+      rootView.open('file.js')
+      editor = rootView.getActiveEditor()
+      rootView.project.addGrammarOverrideForPath(editor.getPath(), gfmGrammar)
+      editor.reloadGrammar()
+      expect(rootView.find('.markdown-preview')).not.toExist()
+      editor.trigger('markdown-preview:toggle')
+      expect(rootView.find('.markdown-preview')).toExist()
+      markdownPreviewView = rootView.find('.markdown-preview')?.view()
+      expect(markdownPreviewView.loadHtml).toHaveBeenCalled()
+
     it "does not display a preview for non-markdown file", ->
       rootView.open('file.js')
       editor = rootView.getActiveEditor()