[PATCH] emacs: invert relation between 'notmuch-send and 'message customization groups

[Austin Clements]

Quoth Pieter Praet on Jan 19 at  8:13 pm:
> 'message contains options relevant to 'notmuch-send, not the other way around.
> 
> Thanks to Austin for suggesting `custom-add-to-group'.
>   id:"20120118184408.GD16740 at mit.edu"

LGTM.