Added two options to improve metadata tagging for use in iTunes

I found a couple of problems with the way iTunes and get_iplayer interacted with the way shows are tagged. The main one was that putting the season number on the season means each season is treated as a different show by iTunes. Additionally adding the episode number to the show name is duplicating another tag.

So, there is now tag_shorttitle option that tags the season name without number and tag_shortname that tags the episode name without a number.