cli: introduce the concept of user defined hooks

Add mechanism for running user defined hooks. Hooks are executables or
symlinks to executables stored under the new notmuch hooks directory,
<database-path>/.notmuch/hooks.

No hooks are introduced here, but adding support for a hook is now a simple
matter of calling the new notmuch_run_hook() function at an appropriate
location with the hook name.

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