add ability to DecryptBuffer the middle of a stream

[Miaourt]

Hello!

I'm in the need of decrypting ranges without using the io.Readers provided, I ended up with this solution by adding an option in the sio.Config. If you have something better that comes to mind, I can draft it too!

I wrote the logic inside dare.go as it looked like the most "idiomatic" way of doing so in this repo (every function opening an AuthDec with the provided sio.Config -> then not touching the sio.Config again)

But if you prefer, I can limit the option to decryptBufferV20() by modifying:

if !ad.finalized {
	return nil, errUnexpectedEOF
}

into something like:

if !ad.finalized && (config != nil && config.EndSequenceNumber != nil && *config.EndSequenceNumber == ad.SeqNum-1) { 
	return nil, errUnexpectedEOF
}

But it's a bit more cryptic imho...

Thanks for your time reviewing this PR!