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).
Yes. Homepage is set per-package, Git is per-version.

For homepage, think "www.meteor.com" or the website of Percolate Studio. For the Git, that's usually the Github URL where the actual code lives.