Vim tags

Eh, so this is somewhat incomplete. What works:

:tag
ctrl-]
ctrl-t

with a proper tag stack (no limit on max length, though). Still to do:
- :tags (list the tags, just an annoying formatting problem)
- integrate with the vim jump list in the proper way (although really, using these together is asking for trouble...)  
- visual mode (should be easy to implement)

Hopefully I'll find time to wrap this stuff up later, but for now this is at least functional.