An alternative (hopefully better) way to specify multiple dirs

Provides a more convenient method to specify multiple dirs instead of
in a comma-separated list.  For example, user can indicate multiple dirs
this way:

  $ rerun -d example_dir -d ../tmp/dir "rake spec"

One advantage of this method is that the user gets the benefit of auto-completion when typing the paths on the command line.

* Also add some unit tests to support new options.
* Old-style comma-separated dirs are still supported.