Add helper function operator.create_filter()

[fishroot]

From @fishroot on December 17, 2018 7:41

e.g.

p1 = lambda seq: seq[0] > seq[1]
p2 = lambda seq: isinstance(seq[2], str)
op = operator.create_filter(p1, p2)

Copied from original issue: frootlab/rian#573