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.
Maybe a better name is `CodeMirrorPosition`?  Cursor is a little too generic and confusing.  Also you should document it as such:
    `...by finding the CodeMirrorPosition Object that matches the `position` argument`.