Only show lint warnings from most recently edited file in browser console

Created by: gaearon

This came up in discussion in https://github.com/facebookincubator/create-react-app/issues/2070#issuecomment-304388814. Seems like a sensible thing to do, although I'm not sure how hard it would be.

Tagging as good first bug but I don't have any ideas on how to do it.