Caching strategies

[JonasWanke]

As @marcelgarus already said in https://github.com/marcelgarus/flutter_cached/issues/5#issuecomment-598095137:

Actually, the current behavior is somewhat intended as a temporary behavior. In the long term, I was thinking about implementing multiple caching strategies that you can choose from (for example, only fetching the first time or fetching only if the last fetch is some amount in the past)