Refactor finding trigger candidates

* gnorb-utils.el (gnorb-find-tracked-headings): New function: take a
  pile of message-ids and return any relevant headings.
  (gnorb-choose-trigger-heading): New function: Ask if the user wants to
  trigger this ID, or else make them pick a different one.

* gnorb-gnus.el (gnorb-gnus-attach-part): Use new functions.
  (gnorb-gnus-hint-relevant-message): Use new functions.
  (gnorb-gnus-insert-format-letter-maybe): Use new functions.
  (gnorb-gnus-view): Use new functions.
Move roadmap/todo list from manual to README

* README.org: Add roadmap
* gnorb.info
* gnorb.org: Remove roadmap
* gnorb.texi