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.
I'm not sure I understand your suggestion of using previous `matchIndex` as the starting point. So you're not suggesting to use `_.findIndex()` which does not take another argument to use the prev match index. Also, as I mentioned in previous comment as an update, this change does uncover one issue that in my prior implementation in replace scenario. So can you show me the code that you're suggesting? And does your suggestion work for replace scenario where the index does not change after replacement, but the total match does reduce one.