Support listing by derived properties

[luser]

Unicode has a concept of "derived properties" (the canonical list is here) which are defined as a combination of other character classes. For example, the first entry in that list is:

# Derived Property: Math
#  Generated from: Sm + Other_Math

It would be nice to be able to pass these as arguments to print.

[arp242]

Sure, this sound useful!

Just to set expectations: I'm unlikely to work on this any time soon myself; although I'll be happy to review and merge patches in a timely manner.