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 think the point of 'show-all' is that it doesn't hide anything. :p 

Sort of, less snarkily (and I am not trying to be snarky here at all, honestly) -- we wanted to simplify the interface to 'meteor show' and not have too many parameters. And, well, sometimes you *do* want all of the information, so there has to be a way to get that. Too many fine-grained controls makes for a convoluted and hard to explain interface.

I think a better solution, in the future, might be to print pre-release versions separately in the main view, for example, like:

Recent versions:
   0.9.0 
   1.0.0

Pre-release versions:
   1.0.0-rc.5

But I don't think that we should volunteer to do that, unless we actually get complaints about it! There is no need to complicate an interface if people don't need the feature.