allow to pass in a custom deserialiser (eg for use with custom registry)

[anentropic]

I've developed a custom registry which extends the one from refract.contrib.apielements with additional element classes and accessor methods, to make working with the refracted output closer to the AST output (e.g. to be able to do like parse_result.api.resource_groups[0].resources[0].transitions[0] instead of lots of anonymous .children)

So to be able to use it with draughtsman I need to be able to pass in a deserialiser which uses the custom registry.

[anentropic]

Hi, I've updated this PR against the current version and I think cleaned it up a little...

So just pinging you again in case it could get reviewed and merged 😄