Allow alternative hash functions

[d3zd3z]

Gosure and friends (rsure, etc) all use SHA1 has the hash function. Since these programs are more oriented toward integrity and not security, it isn't that harmful to use a weaker hash function. But, the user may want to consider other tradeoffs between performance and robustness of the hash function. Maybe they want increased performance, and use something like siphash or xxhash.