Refactor/native regexes

Here's the tasks for 

- [x] Remove `OnigRegExp` and use plain `RegExp` instead.
- [ ] Fix case so that `Color(...)` is not matched by `color(...)` expression.
- [ ] Add uppercase and capitalized versions in the named colors expression (required by the change above).
- [ ] Fix (or workaround) named colors matched even when prefixed.
- [ ] Implement a real asynchronous search with batched loop so that parsing large files doesn't freeze Atom. 
- [ ] Clean up API to remove asynchronous methods when not needed.