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
It's worth using hljs.inherit here: `hljs.inherit(hljs.QUOTE_STRING_MODE, {contains: [SUBST, ... ]}`. This way it'll pick up possible changes to the base mode in the future.