Add 'deadline' and 'warn_only' options

Use cases:
  FIXME 'fix shit', deadline: '2014-01-01'
  XXX 'bad stuff, refactor soon', deadline: '2014-01-01', warn_only: Rails.env.production?
Merge pull request #1 from spajus/deadlines

Add 'deadline' and 'warn_only' options