auto_profiles: add this script

This is taken from a somewhat older proof-of-concept script. The basic
idea, and most of the implementation, is still the same. The way the
profiles are actually defined changed.

I still feel bad about this being a Lua script, and running user
expressions as Lua code in a vaguely defined environment, but I guess as
far as balance of effort/maintenance/results goes, this is fine.

It's a bit bloated (the Lua scripting state is at least 150KB or so in
total), so in order to enable this by default, I decided it should
unload itself by default if no auto-profiles are used. (And currently,
it does not actually rescan the profile list if a new config file is
loaded some time later, so the script would do nothing anyway if no auto
profiles were defined.)

This still requires defining inverse profiles for "unapplying" a
profile. Also this is still somewhat racy. Both will probably be
alleviated to some degree in the future.