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).
Random thought --- Huh, didn't realize we had an ejson isopacket (at some point in time it seemed like that wouldn't be possible or something, that was before some refactorings).  This suggests that catalog-remote could probably use EJSON.stringify to store contents rather than JSON.stringify...   anyway, totally unrelated to this commit :)