Rigify: support executing an arbitrary script after generation.

Just in case the user wants to apply some custom changes to the
generated rig, allow executing a text datablock as a python script
after generation completes.

The script is executed with the generated rig active and in object
mode. When executed by rigify, the generator instance is available
via `rigify.get_generator()`. Outside of Rigify generation the return
value is None.