Proposal: Expirable LRU Disk Cache

**Glide Version**: 4.8.0

With upcoming [GDPR](https://en.wikipedia.org/wiki/General_Data_Protection_Regulation) it would be nice to have some disk cache expiration policy. E.g. I want to cache image for 30 days.

This can be achieved by modifying `DiskLruCacheWrapper#get` method.

```java
@Override
public File get(Key key) {
// ...
if(expired) {
result = null;
delete(key);
}
// ...
}
```

If you think this may be useful I can send PR.