-Ystatistics accepts a list of phases for which to print stats

Also cleaned up handling of defaults in MultiChoiceSetting.
It's only a dozen lines of code, but I know it will be touched again because we'll want to negate expanders, like `Xlint:_,-beginner` to turn off beginner advice.  Possibly the expression in the gist is more clear. (But same refactored logic.)

https://gist.github.com/som-snytt/45b82e58310c8ebb1e5f#file-expander-scala-L33