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).
We played around with wording a lot for this -- part of the issue is that we don't know whether we are asking for a package or a release. So, possibilities would be "unknown package or release" (which see too verbose) or "unknown item" (which is what we had before, but is too technical). So, we settled on 'foo: unknown item'. 

I think the most important thing is to parallel 'meteor search', so if we change one, we should change the other.