code: function hash(password, salt, iterations = 1024, keylen = 32, digest = 'sha256', encoding = 'hex') {
    return pbkdf2(password, salt, iterations, keylen, digest).then((buf) => {
      return buf.toString(encoding);
    });
  }
docs: Hash a password with salt.
  
  @method
  @param {String} password
  @param {string} salt
  @param {Number} [iterations] 1024
  @param {Number} [keylen] 32
  @param {String} [digest] sha256
  @param {String} [encoding] hex
  @return {Promise} by default 64 of password_hash length.