Complete support for monitoring extra files for reloading

[csojinb]

The werkzeug-adapted reloader code has support for adding additional files on which to trigger reload. By default, the reloader looks only for *.py files, but a user may also wish to trigger reload when, say, a configuration file changes.

We would need to plumb through the support for extra files (probably through to the command-line) and add tests.

[vytas7]

A modern/popular alternative: https://pypi.org/project/watchfiles/
(Uvicorn uses its older variant)