Add support for --exclude-dir command line option

This will allow more fine grained control of what directories
will be loaded as part of a spec run.  Currently, all files
matching the default or specified patterns are run, and Dir#glob
will not allow complex enough pattern matching for all cases.

In my case, I need to run all specs under a folder, except
those that are under a subfolder, while also not loading those
files under the subfolder.