Create plugins/enabled directory if it doesn't already exist

- This fixes a bug I saw after installing bash-it on Mac OS X 10.6.8.
  During bash-it installation I chose not to install any plugins. After
  install, when `bash-it enable plugin foo` was complaining that the
  enabled directory did not exist.
Merge pull request #193 from upgradingdave/small-fix-ensure-enabled-dir-exists

Create plugins/enabled directory if it doesn't already exist