Support Cache-Control extensions

[mfr-itr]

I'm thinking specifically about https://bitsup.blogspot.fr/2016/05/cache-control-immutable.html.
Maybe add a dict str -> (str or None) which maps the extension name to the optional payload?

[jab]

@mitsuhiko and other maintainers, would you accept a PR implementing Cache-Control: Immutable (via e.g. response.cache_control.immutable = True)?

[davidism]

This is still marked experimental on MDN, I'm not sure I want to add that yet. Werkzeug already provides the tools to include whatever experimental options you want:

# cache_control is also a dict
# None means set for bool options
response.cache_control["immutable"] = None

# or add the property
from werkzeug.datastructures import ResponseCacheControl, cache_property

ResponseCacheControl.immutable = cache_property("immutable", None, bool)
response.cache_control.immutable = True