Add output line handler
Add output line handler

Summary: Allow investigating output lines in `redex.py`.

Reviewed By: wsanville

Differential Revision: D28515112

fbshipit-source-id: 9686a5c3c105c20c4d70cc6af857dc5b49b4ccc0

facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex