Support display rotation

In some situations, rotating the display 180° is useful. For example, if the HAKS is placed flat on a desk, the default orientation causes the power cord to come out the front (towards the viewer). But in many situations, having it come out the back (away from the viewer) is more aesthetic.

This can be accomplished with a simple addition to the main YAML. Noting this in the README would be helpful for people in that situation.

# Rotate display for power cord back orientation
display:
  - id: !extend haks_display
    rotation: 180°