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).
we apparently check this in the client at release publication time but (a) never check it on the server and (b) never check it (on client or server side) when running recommend-release.  (and we rarely create recommended releases anyway (only for 'backports', not for latest releases), so it's more important to have this check in recommend-release than in release creation!!)