emacs: logically group def{custom,face}s

To allow for expansion whilst keeping everything tidy and organized,
move all defcustom/defface variables to the following subgroups,
defined in notmuch-lib.el:

- Hello
- Search
- Show
- Send
- Crypto
- Hooks
- External Commands
- Appearance

As an added benefit, defcustom keyword args are now consistently
ordered as they appear @ defcustom's docstring (OCD much?).

Proper defgroup docstrings and various other improvements
by courtesy of Austin Clements.