Allow specifying a member group in pk;export

[sisby-folk]

Currently, pk;export takes no arguments, and exports the entirety of the system data in JSON.

It would be useful when utilizing the data elsewhere to be able to only export members that are part of a specific group. i.e pk;export [groupname], which would simply not include any members that aren't present in that group.

Saves us having to edit it manually anyway.