Add a cache management for entities

[LeMyst]

Maybe something like this

wbi.item.load(['Q42', 582])
and then use
wbi.item.get('Q582', use_cache=True)

or maybe something like this
wbi = WikibaseIntegrator(use_cache=True)
wbi.item.load(['Q42', 582])
wbi.item.get('Q582') # Use the cache
and later
item = wbi.item.get('Q2') # Try to use the cache, no cache, fallback to API and update the cache
item.write() # Write the entity to the Wikibase, update the cache
[...]
wbi.ite.get('Q2') # Try to use the cache, found the (updated) entity in the cache, use it

The cache can lead to concurrency issue if someone else edit the entity.

[dpriskorn]

I don't think this is a good idea.
Let's keep WBI simple. If someone wants to cache they can export the JSON representation and save it to disk.
There are python modules like save-to-disk