Support multiple response codes

[duckpunch]

It seems prmd only supports a single response code per link. It'd be great to be able to describe different response codes so that we could document non-trivial error cases.

As with #294, I'm happy to submit a PR if this seems like something that fits the project.

[geemus]

Sounds reasonable. Similar to #294 could you provide explicit examples of what the schema and docs might look like? Thanks!

[duckpunch]

Does the example I provided in #294 suffice? Maybe they can be rolled into one ticket.

[geemus]

They are slightly different, but certainly related. We can continue the discussion there and see where we end up.