Bug: static mode broken by customization

[Wivik]

venv/lib/python3.11/site-packages/flask_frozen/__init__.py:205: MissingURLGeneratorWarning: Nothing frozen for endpoints custom_static. Did you forget a URL generator?
  return set(page.url for page in self.freeze_yield())

Weird, the customization was merged before the last search engine feature and the static mode has been tested for both of them...

[Wivik]

I'm not sure yet, but I think a configuration file instead of letting the code searching for itself should be a better way for maintainability. Working on #6 also made me think it would be a better way.

Also the begin of the code is kind a mess with some settings declaration.