regex support

Midje supports regexes as the right-hand argument of `=>`, testing whether the string on the left-hand side of the arrow matches that regex. It would be nice to also have that in testit.

Start by locating how testit handles the right-hand argument of the `=>` assertion, then compare that behavior with Midje's regex support. Done means a string on the left-hand side can be tested against a regex on the right-hand side, with coverage for the new assertion behavior.

Written by the indexing model from the issue text.