Literal cheap caching with MongoDB - for storing documents from APIs that charge on a by-the-call basis.
Provides a decorator such that if the decorated function's argument has been previously called, it will be retrieved from MongoDB. If the argument is not cached in MongoDB already, both the argument and the results will be cached there.
Can only be used to decorate a function that takes a single argument (such as the API call URL) and returns a string (assumed to be a json_string).
NOTE: This decorator DOES NOT take care of removing cached values from the cache.