Show match index in place of match count

This is the implementation of https://trello.com/c/0GXqt5GF/1089-s-find-next-indicate-current-match-index.
@RaymondLim This might have performance implications though -- now it's O(n) every time you invoke Find Next.  Might not be noticeable most of the time, but it's not ideal.  Do you think it's worth trying my earlier suggestion?  If you use the previous `matchIndex` as the starting point (and then search in the direction the caller passes in as a hint), then it will remain O(1) for most cases.