support `filter` argument in cache decorator

[mattshin]

It would be nice to have an optional parameter such as filter in the @cache decorator that took as input a python callable w/ the wrapped function's *args and **kwargs and returned a boolean as to whether or not to cache.

The default could just be always true e.g. lambda x: True but users may want more nuanced control, based on the parameters to the function itself. In my particular case, I can't quite use cache-control header because i want to determine whether or not to cache on the server side based on query params.

If I get some time, I can jam on this and submit a PR, if you think it could be worthwhile?

[long2ice]

Does key_builder param satisfy you?

[mattshin]

not quite, although I technically could use it to hack a solution by returning a key I know has not been cached. Ideally, the filter callable is called before a key is even built, and if the filter returns false, then the entire caching process (both get and set) is not attempted.

[mjpieters]

I was already thinking the key builder should be able to flag a request as not cacheable, either by returning None or raising an exception here.