Generate metadata slugs

This is an implementation of the metadata JSON generation proposed by #43. I tried to adhere as closely as possible to the original idea. 

To generate a metadata slug of an npm folder, just run `bin/biscotto [folder] --metadata`. It'll figure out the src/lib stuff, as well as the *package.json* main file/dependencies.

@nathansobo Truly, I'm not sure what the best way to test this would be. There are [a bunch of tests](https://github.com/atom/biscotto/tree/slug-format/spec/metadata_templates) demonstrating output JSON samples. Would it be useful to go over these? Or maybe just run the generator on an existing module folder and see what happens?

To be addressed:

- [ ] multiple requires on a single line (eg. `{faz, baz} = require 'kaz'`) (was working at one point but I broke it)
- [ ] row/col numbers are CoffeeScript defaults and are 0-indexed, not 1-indexed. is that okay?
- [ ] missing `doc` key on functions
- [ ] supporting a `main` file that's a JS file, even though it's a CS project

Resolves https://github.com/atom/biscotto/issues/43.