Add dropdown select for # issue tag
Since the library we are currently using does not support multiple
delimiters in the same text area and has a very inactive dev,
change to use At.js.
Example1 (issue): http://i.imgur.com/yZYGamA.png
Example2 (user): http://i.imgur.com/jzh3SFa.png
* Use @ to reference a user
* Use # to reference an issue/pr
* NOTE: when using markdown formatting for headers, the dropdown will
still appear. This is consistent with behavior in github and others
though