Make using requests_cache optional with argument

[seitzbg]

Should be pretty easy to drop in requests_cache flag along with parameters for caching. I have a lot of media to look up and it would definitely speed things up and reduce load on the server side!

Something like this seems to work in client.py:

self.session = requests_cache.CachedSession(cache_name='omdbapi_cache', expire_after=604800)

Obviously values for name/expire/backend could be passed in.