8 byte nonce length

[ChrisCA]

I've a use case for this library but with a nonce length of 8 instead of 12 bytes. I made a (dirty) fork which is sufficient for my needs but I would like constribute upstream if an optional 8 byte implementation is the scope of this library.

[mcraiha]

Sorry about response delay. Yes, if you have a good way to separate the constructors then I will gladly accept pull request (I assume this about original ChaCha20 and IETF variant)