This version fixes the issues mentioned by Austin and highlights the currently displayed message in the outline buffer. My previous issues with 'point-entered and 'point-left were caused by linum-mode, so don't enable it for notmuch-show buffers.
I haven't had time yet to implement a more sophisticated thread structure display as suggested in id:"87y613vpi6.fsf at servo.factory.finestructure.net" though.
