Feat: Hosts `callback` None is part of matching constraint

[nitsujri]

Request

If callback_func in hosts(callback=callback_func) returns None, then it continues checking the host_patterns.

This allows callback_func to act as part of the pattern-matching constraint.

Why

• If there are no matches, automatically returns 404 instead of requiring manual handling inside the callback.
  • Currently, the callback has to manually return a HttpResponse() as None will continue the request.
• Allows for DB based pattern matching.
  • Complex apps/lots of routes can have stronger sub-grouping.

Possible Alternatives

• Add a constraint_func to hosts() instead.

Next Steps

I'm happy to work on it, but wanted feedback first since it should be considered a breaking change.