mrhlpr, mrtest: Add --version command

This also effectively reverts 7d068e20, because now we actually use the version for something in the program. It also avoids the problems reported in #15 (closed) to the best of my knowledge — I tried reproducing the bug as written in there and did not get any build failures.

The primary purpose of this version command is for making experimentation with mypyc easier, but I also can imagine it might be nice for users to be able to get their version of mrtest easily.