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).
Yeah I think the number of files in tools/ is kind of crazy. That would probably be a separate PR from this stuff, though.

Also, maybe it's worth it to separate the actual command definitions from the logic behind them? Right now it's really hard to scan those files and find where the actual commands are defined.