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).
So is meteor show intended to be a replacement for atmosphere searching? If so, it seems like it should ignore local packages? Otherwise we should make it explicit that if you run meteor show from an app, it will return different things than if you run it outside.