Add snap packaging

[farshidtz]

The current systemd service is useful on Raspberry Pi Zero W (armv6) on which snaps aren't supported. Adding a snap is useful for running the service on armv7 and armv8 versions of the Raspberry Pi, to allow automatic updates and additional snap features.

The run.sh script currently sources an env file. It can be modified to read snap options when executed by a snap, e.g. by checking if SNAP is set, and export the equivalent environment variables.