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).
I feel like this raises questions that it doesn't answer.  At the risk of making the code more complex, it could say one of the following:

- Older versions of N have been hidden
- Pre-release versions of N have been hidden
- Older and pre-release versions of N have been hidden

etc.  (i guess unmigrated too, sigh)