Feature request: Support XChaCha20_Poly1305

https://tools.ietf.org/html/draft-irtf-cfrg-xchacha-01#section-2

```
AEAD_XChaCha20_Poly1305

   XChaCha20-Poly1305 is a variant of the ChaCha20-Poly1305 AEAD
   construction as defined in [RFC7539] that uses a 192-bit nonce
   instead of a 96-bit nonce.
```