Add helper for markdown extension to fs

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

@@ -70,5 +70,4 @@ class MarkdownPreview extends ScrollView
     @markdownBody.show()
 
   isMarkdownFile: (path) ->
-    extension = fs.extension(path)
-    extension is '.md' or extension is '.markdown'
+    fs.isMarkdownExtension(fs.extension(path))

src/stdlib/fs.coffee

@@ -151,3 +151,12 @@ module.exports =
     _.contains([
       '.pdf'
     ], ext)
+
+  isMarkdownExtension: (ext) ->
+    _.contains([
+      '.markdown'
+      '.md'
+      '.mkd'
+      '.mkdown'
+      '.ron'
+    ], ext)