[Collection|List]Utils.[intersection|union] should accept vararg parameters

Would be convenient if intersection/union methods accepted arbitrary number of arguments. Two-arg methods could be retained in order to avoid compiler warnings (https://issues.apache.org/jira/browse/COLLECTIONS-481?focusedCommentId=13762173&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-13762173). Alternatively, method could be overloaded to accept a single Iterable of Lists/Sets. I could create a patch for this if it's deemed worthwhile.