Show revamp

Complete rewrite of 'meteor show', with tests. See commit messages for more details.

This is part of the work on the Meteor tool to get longform package descriptions from README.md files into 'meteor show'. It is safe-contained enough, that it makes sense to submit it for review.

(I won't merge to devel until all the README work is complete -- note that this is being pulled to a staging branch).
I feel like I've never been sad to have to read an actual name next to an argument instead of just a list of three things. Our style guide does say "Keep the most important arguments in the signature" though. I tend to err on the side of putting things in options, but that's just my taste.