Only show markdown preview if the edit session's grammar is GFM

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

@@ -15,6 +15,8 @@ module.exports =
       return
 
     editSession = item
+    return unless editSession.getGrammar().scopeName == "source.gfm"
+
     if nextPane = activePane.getNextPane()
       if preview = nextPane.itemForUri("markdown-preview:#{editSession.getPath()}")
         nextPane.showItem(preview)

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

@@ -4,6 +4,7 @@ MarkdownPreviewView = require 'markdown-preview/lib/markdown-preview-view'
 
 describe "MarkdownPreview package", ->
   beforeEach ->
+    window.loadPackage('gfm')
     project.setPath(project.resolve('markdown'))
     window.rootView = new RootView
     atom.activatePackage("markdown-preview", immediate: true)
@@ -17,6 +18,14 @@ describe "MarkdownPreview package", ->
       beforeEach ->
         rootView.attachToDom()
 
+      describe "when the edit session does not use the GFM grammar", ->
+        it "does not show a markdown preview", ->
+          rootView.open()
+          expect(rootView.getPanes()).toHaveLength(1)
+          rootView.getActiveView().trigger 'markdown-preview:show'
+          expect(rootView.getPanes()).toHaveLength(1)
+
+
       describe "when a preview item has not been created for the edit session's uri", ->
         describe "when there is more than one pane", ->
           it "shows a markdown preview for the current buffer on the next pane", ->
@@ -76,4 +85,4 @@ describe "MarkdownPreview package", ->
           preview.fetchRenderedMarkdown.reset()
           pane1.saveActiveItem = () ->
           pane1.trigger("core:save")
-          expect(preview.fetchRenderedMarkdown).toHaveBeenCalled()
+          expect(preview.fetchRenderedMarkdown).toHaveBeenCalled()