Add support for user defined transformations

[maandree]

The reason the advance parameter is included in the hook's parameters is to allow the glyphs to be mirrors around their center.

[tomolt]

Hi Mattias!

It's great that you took the time to implement this, but I'm really not liking the changes to the interface this introduces. In the last couple of days I experimented with some alternative implementations of this, but I still haven't decided yet. For now, I think it's better if I concentrate on some of the other things I've planned first.