Add `loader` support to `django_router`

[Archmonger]

Description

Adds support for the loader arg within route.

Checklist

Please update this checklist as you complete each item:

• [ ] Tests have been developed for bug fixes or new functionality.
• [ ] The changelog has been updated, if necessary.
• [ ] Documentation has been updated, if necessary.
• [ ] GitHub Issues closed by this PR have been linked.

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[Archmonger]

This is going to be better done within reactpy_router.route(loader=...), so I'm closing this PR.