* @param {string} apiKey - devtodev API key, unique API key can be found in the application
* settings ("Settings" → "SDK" → "Integration")
* @param {string} userId - Unique user identifier.
* For example, user’s ID in a social network, or a unique account name used
* for user identification on your server.
* @param {string} previousUserId - Previous unique user identifier. Optional.
* It is used in case of change of the user identifier.
devtodev.init(apiKey, userId, previousUserId);