Add option in preference to allow user import tracker lists online

[lumingzh]

Some BitTorrent apps allow user subscribe and import tracker lists online from like https://ngosang.github.io/trackerslist/ or similar.

There are a lot of torrent files and magnet links without trackers, or with trackers banned. So users have to find useful trackers to add to torrent files and magnet links before download.

This option will allow Varia auto add imported trackers to torrent files and magnet links to help users complete the download. Thank you.