Add zero-width space character before and after matches for emoji aliases

make it so that :)P doesn't evaluate to :smile:P (which then evaluates to :smile:tongue:, which gets shortened to :smile: tongue:).

Fixes #1054

Insert zero-width spaces before and after substituted tokens.
![croppercapture 5](https://cloud.githubusercontent.com/assets/1271535/3487085/c095ce2a-0462-11e4-8ebb-0e10a10766b9.jpg)