Export xit and xdescribe

[chmac]

I haven't tested, but I think that just adding them to package.js as exports would do the trick. They seem to already exist (at least as far as my GitHub / browser find investigation went!) and so should "just work". :-)

If I get around to testing this myself, and it works, I'll post a PR.

PS> Thanks for mocha testing in packages, it's simply #awesome! 👍