Added functionality to get single episode(s) from a season torrent

- If the best (or only) available episode(s) is in a season torrent
  the episode(s) file(s) will be selected for download from the
  torrent while all other files will be set to "skip/ignore".
  (Multi-episode torrents will also be split if not all episodes
   are wanted)
- This functionality is only available if both the torrent provider
  and the torrent provider has the required functionality:
    - The torrent provider must support retrieving a list of the files
      in the torrent (supportsTorrentFileList)
    - The torrent client must support specifying what files should
      be downloaded/skipped (supportsFilesWanted).
- The required functionality has been added to the Transmission
  client and the torrentleech and TvTorrents providers
- As with the existing nzb release/season splitter functionality
  this only works when setting a episode(s) to the status of
  "wanted". Using the per episode search button will not work,
  though I am looking into fixing this in a generic way for both
  nzbs and torrents and merge it upstream.

Notes about code changes:
- The Transmission client now recognizes an add "duplicate torrent"
  event and will try to send a new request specifying which extra
  files in the torrent are wanted where available.
- For the torrentleech and TvTorrent providers, in the
  "_get_episode_search_strings" methods I have removed code which
  adds specific episode search strings if not all episodes in the
  season are wanted. This is an optimization to remove unnecessary
  searches as the season search strings (i.e. "ShowName S01)
  should catch all episodes in that season anyway. I recommend
  doing the same with the other torrent providers, and in the case
  that they have too low a number of results per page instead add
  support for iterating the result pages.