Add support for custom Snippets

[oliverhr]

Sometimes I use the TEMP tag or DEBUG, it would be nice if we can add custom snippets, and I think this can fix issues like https://github.com/atom/language-todo/issues/58 if the plugin just handle by default the most used tags like TODO and FIX so users can add its own tags/snippets.

[guywaldman]

Would be very much appreciated!

[dillanjwilding]

I agree! This would be very helpful when working with teams who majority uses another IDE.

[graphicagenda]

+1

[Loschcode]

+1

[wismill]

+1