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).
Well, it exists locally. Like, there is the version of the package that we are showing to you (and you can tell that it is local, because we are showing you its source directory), and there is another version of this package with the same version number on the server. Otherwise, we have to explain what a 'local package' is.