Add LiPO battery support

[jkaczman]

It would be interesting to develop functionality to optionally support a LiPO backup battery, where we could read the capacity percentage left in-app. It would allow storing motion videos locally in case of a power outage, which would then be sent over after re-gaining Wi-Fi.

Make DIY guide on how to do this.

[jkaczman]

If paired with #24, could indicate whether or not it's actively on backup power in the app too. Perhaps with a notification.