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 is an interesting thing to look into: http://www.2ality.com/2013/02/foreach-es6.html (parameter desctructuring)

@benjamn, what's your thought on ES6 named parameters? Are they in the standard? Can we cross-compile them easily?

I have no problem with named arguments, I just think that in the current implementation via a single "options" argument, there isn't enough IDE/editor and language support to make the arguments really visible - you have to read the doc comments (which can easily become out of date) or the code.

Also, if we stay with the options object approach, can we use the standard JSDoc style to comment the options, like here: http://usejsdoc.org/about-getting-started.html  Then, maybe we can hack in some editor support to see the actual function signature. Also, many editors have built-in support for this format of comment.