Add supermarket_attributes to Metadata

Allow the metadata.rb in a cookbook to specify arbitrary attributes for
use from Supermarket. The usecase for this right now would be setting the issues
and source URLs for that Cookbook instead of specifying them in the
Supermarket UI.

@tristanoneil and I paired through making this happen.

You can verify we are authorized to contribute:
- https://supermarket.getchef.com/users/tristanoneil
- https://supermarket.getchef.com/users/brettchalupa

This is our first time working on the chef codebase, so if there is anything that needs
to be changed or added, just let us know.

I uploaded a test cookbook to Supermarket Staging entitled Salsa with a specified
`source_url`: https://supermarket-staging.getchef.com/cookbooks/salsa/versions/1.0.1
You can verify it works by checking out the metadata.json in the cookbook tarball.

Thank you! :sun_with_face:
This should be a Mash, for consistency, as the other attributes in the metadata are.