Add allow-test-packages option - #12

This needs a change to the README to document the new option.

Is there a package that is only available when --allow-test-packages is enabled?

There are usually a few, but you can't rely on the set remaining constant, or having any members.

I'm not totally sure how setup behaves if you explicitly name a package on the command line, which only has a test version available. I think it gets installed, even if -t isn't specified.

So, I guess a test that the new option is accepted is the best you can do. Oh, no, I guess it could be combined with a test which installs from the CTM, and pick a package which has a test version and check that version is installed?

Merged, with requested documentation addiiton.

Thanks for writing this. Sorry about being so incredibly slack about merging it!