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'd be a fan of the tool directory developing some sub-directory structure in general: our files, and in particular, the commands files are kind of stupidly large, IMO. (And then we can even try having unit tests!) I think some of that is just a style thing though.