Encrypted disk cache with Glide 4.0

**Glide Version**: 4.0

**Issue details / Repro steps / Use case background**: Encrypted disk cache

Hi,

I have been trying to add encryption to Glide disk cache. But it's not clear how to do that with Glide 4.0. I've implemented Encoder\ and ResourceDecoder. I suppose that I should register them in AppGlideModule.

I found out that I can register Encoder like this

`register(InputStream::class.java, SecureEncoder())`

But what should I do with SecureDecoder to decode images from disk cache?

Please let me know if I'm on the right direction. It would be nice to have an example how to encode/decode images on disk cache.

Thank you.