Pool resize option

This is a basic grow/shrink option for the pools.

I'd like to improve it on the completion part:

1) if one would type "nipap pool resize X remove" it should complete the command with a listing of prefixes that are in that pool. Can I supply arguments to the complete_something method?

2) I'd like it to spit out that the argument to add and remove are obligatory: Not supplying them with this code is now silent.

Regards,