Add user-defined regex option

I wanted to use GutterColor in a few situations not included already (e.g. quoted hex strings without initial #), but I figured they weren't common enough to merit being a default search. So I implemented a `custom_colors` setting that takes an array of objects defining new regexes and trivial string modifications to allow everyone to define their own "nonstandard" searches without modifying the package core.

![custom_regex_settings](https://cloud.githubusercontent.com/assets/1791252/3773924/53690cf8-191c-11e4-98e9-3282f568820d.PNG)

![custom_regex](https://cloud.githubusercontent.com/assets/1791252/3773918/224d9fd0-191c-11e4-9afb-6aed81eba62a.PNG)