Add option to shuffle test files.

This is an idea that I'd like to put forth:
Add "shuffle" option. This adds a -u/--shuffle option (default is "do not shuffle", as it is currently).

Reason:
I really do like the new -a option. However, I do believe - ymmv - that tests should be run in random order to detect setup/code loading issues.

Currently, I do have to use something akin to `FileList["spec/#{dir}/*_spec.rb"].shuffle.join ' '` to make this happen, which negates the benefits of `-a`.

Always shuffling, adding a shuffle option (e.g. `-u`), or always shuffling on -a would be my wish-order.

I'd like to put this idea up for discussion. Thanks for considering it :)