Simplify and streamline style pre-processor support

- simplify bootstrap support greatly. Choosing less is now the
  first option(happy path), with note in prompt that says choosing
  less will include bootstrap. no other pre-processor options deal
  with bootstrap either

- add default base.{less,scss,styl,css} based on the pre-processor chosen

- [x] remove commented code in generator support for bootstrap

fixes #60, #12, #44, #46

(included here cause a new PR would just complicate things further)