[unstable option] binop_separator

[scampi]

Tracking issue for unstable option: binop_separator

[aloucks]

Are there any unresolved questions or work needed for this option to be stabilized?

[emilio]

Please consider stabilizing this? Servo has been using it since forever, and it's been stable as far as I can tell.

If there's no issue with this option it'd be nice to be able to use stable rustfmt.

[karyon]

This will need to be re-done for 1.x