Initial swift support

This is a first stab add adding Swift support... not thoroughly tested yet. Some things that need to be done:

- [x] Nested comments
- [x] Test Generics
- [x] Add Contributor changes
- [x] Shorten example
- [x] Remove JS leftovers
- [x] Split lines that are > 80 chars
- [x] Remove empty category
- [x] Remove relevance
- [x] String interpolation
I suspect the SUBST can contain more than just keywords, so it's worth to add a "contains" attribute to it after the definition of the QUOTE_STRING_MODE: `SUBST.contains = [QUOTE_STRIING_MODE, hljs.C_NUMBER_MODE, <something else?>...]`