completion: complete bash completion rewrite

Rewrite the bash completion script to actually do something
useful. Supported completions:

* All the notmuch commands, command line arguments, and values for
  keyword arguments.

* Tags after + and - in 'notmuch tag'.

* Config options in 'notmuch config', and some config option values.

* Search prefixes in all commands that use search terms.

* Tags after tag: prefix in search terms.

* User's email addresses after from: and to: in search terms.

This is all based on the bash-completion package [1], and will not
work without it.

[1] http://bash-completion.alioth.debian.org/