code: function getKey(key, masterPassword) {
      var contents = getContents(masterPassword);
      if (contents[key] != undefined) return contents[key];
      else throw new Error(`Unable to find key '${key}' in password safe.`);
  }
docs: Get a specific key value from safetext file.
  @param key
  @param masterPassword
  @returns {Promise.<TResult>}