Support for a precomputed x

[jinchung]

The current Client class and createSaltedVerificationKey function take in a username and password and will compute an x for you. In some cases, a user may want to provide their own x (for instance, if a user wants to generate an x using a 2-secret key derivation (2SKD) function which requires more than just a password to derive the x, or perhaps some other more involved algorithm for generating an x).

I'd like to submit a PR for additionally supporting a user-provided precomputed x, with all else remaining the same.