Fix specific hashing options

[iegomez]

When building a hasher we may pass a backend to check for specific options. For Postgres, docs say the prefix should be pg, just as in other Postgres related options, but here we are passing postgres instead, resulting in specific hasher options being discarded: https://github.com/iegomez/mosquitto-go-auth/blob/master/go-auth.go#L257
This may be true for other backends too.

The solution would be to have a backends map name => prefix and derive the correct prefix to pass to the hasher from it.