allow flags to be scoped to a plugin

Add new FlagSet class and init each Path with one.

Call Path.flagSet.addFlag(scope, flag) instead of Path.addFlag(flag)
Merge branch 'expt_add_path_flags_and_git' of github.com:scrooloose/nerdtree into feature/new_expt_add_path_flags_and_git

* 'expt_add_path_flags_and_git' of github.com:scrooloose/nerdtree:
  allow flags to be scoped to a plugin
Add extra params for RefreshNotifier listener; Add listener for refresh path