Add new regular expression syntax groups

I've been doing work to extend [vim-clojure-static](https://github.com/guns/vim-clojure-static)'s regular expression syntax and thought I might as well port over some of the work to this project since I use it. This adds new syntax for matching regular expression boundaries, back references, zero-width modifiers, quantifiers, and grouping. All new groups have been linked to `SpecialChar` with the exception of `jsRegexpGroup` which is just linked to `jsRegexString` (because linking to anything else makes things complicated).

Also, `jsSpecial` has been patched to support null and control characters. It's also been patched to drop what I'm guessing were octal escape sequences (they're deprecated AFAIK).