[XenForoBridge] Add new bridge
[XenForoBridge] Add new bridge

Adds a bridge for forums powered by XenForo (see https://xenforo.com).

Support between forums may vary due to ever changing versions with no
clear distinction. Especially timestamps may not work depending on the
supported language (should currently work on en-US and de-DE).

Tested on

- https://xenforo.com/community/
- http://www.ign.com/boards/

Notice: XenForo provides RSS feeds for forums (but not specific topics).
For example: https://xenforo.com/community/forums/-/index.rss

ORelio/rss-bridge,RSS-Bridge/rss-bridge,Frenzie/rss-bridge,kranack/rss-bridge,kranack/rss-bridge,RSS-Bridge/rss-bridge,kranack/rss-bridge,ORelio/rss-bridge,kranack/rss-bridge,Riduidel/rss-bridge,RSS-Bridge/rss-bridge,sebsauvage/rss-bridge,Riduidel/rss-bridge,ORelio/rss-bridge,simonbru/rss-bridge,simonbru/rss-bridge,Riduidel/rss-bridge,Frenzie/rss-bridge