Support for co-authoring

[mdtro]

It'd be pretty rad if we could co-author posts so credit is shared amongst a few key people that worked on an initiative or co-wrote the post. :)

[dcramer]

I think doing something like co-author would be too expensive, however simply adding a way to tag a bunch of collaborators would be fairly clean. Just a simple tag-like input box that lets you auto complete (or enter) peoples email addresses. We'd need to support when they're not signed up yet, and reconcile later.