userHighlight.autoColorUser: improved color selection, selectable strategies

With [thanks to](http://www.reddit.com/r/Enhancement/comments/1u5373/feature_request_have_the_user_highlighter/cef5xar?context=3) /u/Rangi42, modules['userHighlight'].options.autoColorUser is improved to select less glaring colors.  modules['userHighlight'].options.autoColorUsing allows users to select the original color selection strategy.