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 agree this would be a nice improvement for another PR.  I'd even argue that we should do one command per file!  `tools/commands/test-packages.js` etc.  (I'd be OK with a `tools/commands/misc.js` for all the random little things like `--version` but any real command with more than 4 lines of code should get its own file.  And we could have eg `tools/commands/run-and-debug.js` or something for pairs of commands that are almost all the same code.)