Turn on flashlight when volume button is held

I have Nexus 4 on Android 5.0 Lollipop, and I'm using Tasker. Is there any way to get the Flashlight to turn on when screen is off and the volume up or down button is held for 2-3 seconds? Then turn off the flashlight if the button is held again. I know there's a %SCREEN variable for the screen part, but how can I detect if a volume button is held?