I need more flexibility with regard to skipping checks, for Datasette: simonw/datasette#1377

An optional callback function that gets the scope and specifies if it should be able to skip checks or not would be a great way to do this, and could be backwards-compatible since it could be implemented as a new optional keyword argument.

app = asgi_csrf(
    app,
    signing_secret="secret-goes-here",
    should_protect=lambda scope: True
)