Add support for router converters

[pimuzzo]

A route with a converter like this:
api.add_route('/{some_field:mc}', some_resource)
produces this kind of spec:
"/{some_field:mc}" instead of "/{some_field}"

https://falcon.readthedocs.io/en/stable/api/api.html#falcon.routing.CompiledRouterOptions.converters

[alysivji]

Hi @pimuzzo. Would love help getting this feature implemented

I don't use Falcon on a regular basis, but am able to review and merge PRs.