Add integration with requests-cache

[JWCook]

I'm currently maintaining requests-cache, and the iNaturalist API is a use case I specifically had in mind for it. Requests-cache can already be combined with pyinaturalist (via requests_cache.install_cache(), which I've already been doing in some of my other projects), but I think this is useful enough to integrate into pyinaturalist directly, using CachedSession instead of install_cache().

Expiration can be configured per endpoint, so one of the tasks for this will be adding some sane default config for the most commonly used endpoints. This should also be user-configurable, with the option to disable it completely.

This will be especially useful for infrequently-changing data like taxa and places, and to a lesser extent observations (although with a shorter cache expiration).