Add support for junit XML result output

[bos]

I use test-framework's support for dumping test results into a junit-compatible XML file in my continuous integration setup.

Since doctest produces no output in this format, I have no handy stats on how many doctest tests I have. This would be very useful to have.

[sol]

I agree that this would be useful. Not sure when I would have time to work on this, but patches are welcome of course. Reporting (/src/Runner.hs) is a little bit messy. If we end up refactoring that, then we should do it in a way that allows parallelization (as long requested by @ekmett).