Bootstrap args WINDUP-376

Instead of doing it in scripts (WINDUP-346), this processes args in Bootstrap class.

If there's some refactoring and then the actual changes, do you want that to be 1 commit or 2?
Windup is Forge addon, so to boot Windup, booting Forge is needed.
To get all the arguments even from rulesets in external dirs, I asume that Windup has to run this Windup-specific discovery. Hence, I would have to separate the code which does this scanning, so it can run outside the `windup-migrate-app` operation.

Is that correct?