Introduce a standard scheme for -Z flags based on existing -X and -Y flags

[propensive]

Many new features of Typelevel Scala will be "hidden" behind compiler flags, which will behave like the existing -X and -Y flags of mainline Scalac. We need a standard way of defining and implementing these.

[puffnfresh]

PR #1 includes a way to list these options:

puffnfresh@d7be78d

[propensive]

That was, like, three whole days ago... ;)