Yeah, this is a fine method – exporting an implicit constructor.

The problem I see here is that there is no way to add new serializers (which we do in openbadger), or replace the default `req` and `res` serializers because `opts.serializers` gets unconditionally overwritten in line 12. Might be better to pull in `underscore` and use `_.default` to do a safe object merge against `opts.serializers`.