Rigify: store advanced options in armature instead of window manager.

By storing the Rigify advanced generation options (name, target rig,
target ui script) in the armature data instead of the window manager
as before, multiple rigs can have different options. Additionally,
these options are stored in the blend file, and not lost when reloading.

Also, the rig name is not automatically suffixed with `_rig`, which
doesn't make sense as far as I can tell.

Differential Revision: https://developer.blender.org/D5675