Twitter plugin

Oi,
  as I mentioned in the email, this PR depends on incremental compilation. Also there are a couple of thing I'm unsure of:

- `*tweet-format*` depends on private accessors. Maybe accessors for documents should be exported.
- The format for `*tweet-format*`, I just wanted something quick and to not reinvent the wheel but I'll appreciate any feedback on how to improve.
- Should configuration errors signal a condition? Do I bundle the condition with the plugin or as part of coleslaw?

I'll be adding the usage documentation and the plugin-conf-error class today or tomorrow