Add hook_user_config

This is what our fork has been using to get per-recip config values for some years.  There's been some question as to whether the hook_config API could be extended instead.  I haven't written any unit tests yet, but will do so if we don't decide I'm rewriting the code first.
In every other QP plugin we have the shebang line set to: #!perl -w

We'd prefer this to be the same.