docs: function findByUsernamePassword(username, password, strict, callback) {
    if (typeof strict === 'function') {
      callback = strict;
      strict = true;
    }
  
    return this.findByUsername(username, strict, ok(callback, (user) => {
      if (!user) {
        return callback(null, null);
      }
  
      user.comparePassword(password, ok(callback, (isMatch) => {
        if (!isMatch) {
          return callback(null, null);
        }
  
        callback(null, user);
      }));
    }));
  }