Pass arbitrary WhiteNoise configuration attributes

[florimondmanca]

Is your feature request related to a problem? Please describe.
Static files are served with WhiteNoise (see staticfiles.py). This library has a few configuration attributes, but there's no way to pass them down the WhiteNoise constructor: we're stuck with the default configuration.

Describe the solution you'd like
Allow to pass WhiteNoise configuration attributes when instanciating an app:

app = App(
    static_config={"max_age": 30}
)

Describe alternatives you've considered
/

Implementation ideas

• Add **kwargs to the static (staticfiles.py) and pass them to the WhiteNoise constructor.
• Add a whitenoise_config kwarg to App constructor, defaulting to an empty dict, and pass it to static.

Additional context
Uncovered by #179