Updated the prompts.

   Rather than prompting "y/n" we prompt for "(y)es, (no)?" in
 the prompt_yn function.

  For the reply() & compose() primitives we now prompt:
    Send mail: (y)es, (n)o, or (a)dd attachment

  Inspired by the code in #87.
  Two new primitives: index_offset() + maildir_offset().

  These return the current offset into the message-list, and
 maildir-list respectively.

  Useful for iterating over messages/folders from the current
 position.

  Closes #113.
  New hook: on_mode_change()

  Called when the mode changes.
  Sync recent changes.

  I had intended to push a release tonight, now I'm back home.
 It'll wait until tomorrow now.