WIP: v0.3.0

[TheTinkerDad]

• Added validation for sensor settings.
• Added a way to define defaults for sensor settings.
• Added configurable update interval for sensors.
• Implemented a built-in "version" sensor
• Implemented fixed value sensors
• Made it possible to run Sensible as a non-root user
• Implemented support for generating .deb packages of Sensible
• Sensible installed via .deb package is now started out of the box as a systemd service