custom hooks can now provide an external init_bugzilla helper when invoking
the gitzilla.hooks.{post_receive,update} hooks. The init_bugzilla function
is passed the bugzilla URL, the username and the password and is expected
to return an instance of bugz.bugzilla.Bugz (from pybugz).
This may be used, among other things, to extend the range of auth options
available to other than those provided by gitzilla. For example, one could
easy implement HTTP Basic auth.
For custom hooks using gitzilla.hooks.post_receive and
gitzilla.hooks.update, cookie auth can be used now. The cookie file is
generated by pybugz and is at ~/.bugz_cookie
