Persistence

It would be great if bigcache had a feature where it could save it's state to disk so it could be restarted. I'm thinking there could be a storage interface which defined the API and then the initial implementation could be some base layout, like JSON or something.

```go
type IStorage interface {
Prepare() error // lock the entire cache.
Save() (bool, error) // save the state to disk, return ok, error
Load() (bool, error) // load from wherever, return ok, error
}
```

This has come up a few times and it might be an interesting interface to explore.