We don't have this feature in the current V0.3.1 software, but it is planned for the upcoming 0.4 update.
However, it is possible to do some similar things:
-To make the camera automatically start recording right after finishing a save, you can enable Auto Record in the Util menu.
-To record video only while holding down the trigger, you can enable Gated Burst record mode.
If you're just looking to conserve battery life, just use Auto Record - recording actually takes no more power than sitting idle.
