Feature/due

Introduce a "due" feature. Like so:

      $todotxt due

      Due today (2012-12-12)
      3. 2012-12-12 Buy GTD book @amazon +wishlist

      Past-due items
      5. 2012-11-11 Find new todo-tool +todotxt

      Due 7 days in advance
      1. 2012-12-17 Install todotxt @cli +todotxt
      4. 2012-12-19 Evaluate installation +todotxt