deluge plugin: config option to add additional trackers

Magnet links in thepiratebay.org's rss feed are missing the tracker links.
The issue is reported, but not resolved.
This extends the deluge plugin to allow addition of trackers upon adding the torrents to deluge.

See here:
https://forum.suprbay.org/showthread.php?tid=139805
and here:
 http://flexget.com/ticket/1831#no3