Optionally allow removing rel="ugc" from links

Following on from #3 (closed), there may be situations where we don't want to mark links in bbcode as UGC: for instance if we use bbcode for our own editorial content or if a user is "trusted" in some way such that we're willing to let them "speak for us." It would be nice to have an option that we can pass to the bbcode renderer to suppress rel="ugc" on links in those cases.