Allow to easily cache the auth token (extending the class)

[forfolias]

According to documentation the token should get refreshed:

• no more than once every 20 minutes (TooManyProviderTokenUpdates Exception in send_bulk_message)
• no less than once every 60 minutes (TooManyRequests if using the same token again and again)

So maybe it is a good practice to allow easily cache the token extending the class and implementing the get_token_from_cache and set_token_to_cache functions.