cli: add support for pre and post notmuch new hooks

Run notmuch new pre and post hooks, named "pre-new" and "post-new", if
present in the notmuch hooks directory. The hooks will be run before and
after incorporating new messages to the database.

Typical use cases for pre-new and post-new hooks are fetching or delivering
new mail to the maildir, and custom tagging of the mail incorporated to the
database.

Also add command line option --no-hooks to notmuch new to bypass the hooks.

Signed-off-by: Jani Nikula <jani@nikula.org>