Add RO crate format

[bentsherman]

PR #3 was originally going to add an RO crate manifest alongside the BCO manifest, but it turns out the RO crate example I was working off of was outdated. There is now the Workflow Run Crate standard, which is an RO crate standard that appears to be similar in substance to BCO. There are multiple "profiles", with the Provenance Run Crate being the most detailed.

We can add a separate output format for this standard. We will also add (in a separate PR) the ability to render multiple formats in the same run, so that users can render BCO and RO crate if they want.