Individual File Approval

It'd be nice if there was some way to mark that I have approved an individual file of a pull request. This doesn't need to be visible to everyone involved in the request; just to the user doing the approval.

I'd like this feature because there are times when pull requests require changes (get denied) and then require approval again. Instead of having to remember what files or chunks of data have been approved I'd like to be able to mark files as "approved."