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).
This returns a list of os architectures: either ["os"] or some combination of [  "os.linux.x86_64",
"os.linux.x86_32", "os.osx.x86_64" ]. We don't want to surface this information in general, but it makes sense to surface it *somehow*, so people can debug issues like 'I can't run Bob's package because he forgot to publish for that architecture'.