Add auto-shutdown feature

Hi.

First thanks for this awesome piece of software.

When the system gets shutdown from xochtil (after being in sleep mode for X hours), the reMarkable isn't shutdown but xochtil is closed and the remarkable get back to the oxide screen, which goes into sleep mode. The reMarkable is never shut down, draining a lot of battery.

Would it be possible to add a configurable auto-shutdown feature so that after X hours or minutes in sleep mode, the remarkable is shut down automatically?

If this feature is implemented, the ability to easily setup a custom screen when the remarkable is off would be great.

Thanks!