Add support + doc example for using mongita with MongoDB backend

[JWCook]

mongita looks like a nice alternative MongoDB client that uses local storage only, with no server setup required (similar to redislite for the Redis backend). It would be nice to add an example to the docs on how to use with with the requests-cache MongoDB backend (and possibly some small compatibility changes on this end).

After testing this out, it looks like at least a couple upstream changes would be required:

• Support projection parameter: Retrieving documents with specific fields with find() is not working/implemented. scottrogowski/mongita#26
• Make replace_one() behavior consistent with pymongo: replace_one() with upsert=True creates a new ID instead of using ID from filter scottrogowski/mongita#34
• Nice to have: support estimated_document_count()
  • I could update requests-cache to fall back to count_documents() if this isn't available.

If those were taken care of, I believe using requests-cache with mongita would just look like:

from mongita import MongitaClientDisk
from requests_cache import CachedSession, MongoCache

backend = MongoCache(connection=MongitaClientDisk())
session = CachedSession(backend=backend)