add string cache in get_string()

Implemented a simple dict-based cache in the get_string()-method.
Not sure if it has any noticeable speed improvement but retrieving a string in a for loop feels wrong and retrieving (multiple) strings before the loop in the add-on code looks ugly.

Example usage where it could have a speed improvement: https://github.com/dersphere/plugin.video.itunes_podcasts/blob/master/addon.py#L172