Support keyed BLAKE2

[mithrandi]

This is probably more of an API issue than an implementation issue, since the current hash algorithm APIs make no provision for keying.

[vincenthz]

This is typically API available in Crypto.MAC. The C files already support the functionality:

e.g.

int blake2b_init_key( blake2b_state *S, size_t outlen, const void *key, size_t keylen )

It just need to be exposed