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).
Similarly, this is doubly irrelevant: these contexts aren't making it to the saveChangedMetadata stage anyway, and even if they were, whatever, it's a temp directory, it doesn't really matter (other than very minor performance concern, which again is irrelevant since it doesn't happen anyway) if you overwrite `.meteor/versions` or not