Allow functions to have a ? suffix, ruby style

Implemented after suggestion in https://www.agileplannerapp.com/boards/105200/cards/7873

I'm not sure if this is a good idea for Smartdown, but i figured it was easier to
discuss with a working code example.

Good: Familiar to ruby devs, allows plugin defs in ruby to use normal naming
Bad: A little confusing in smartdown, as trailing ? has semantics for a named predicate
Merge pull request #78 from alphagov/plugins-with-question-marks

Allow functions to have a ? suffix, ruby style