0.2.1

• Moved nyx-theme and hemera-theme to their own repos:
  nyx
  hemera
• Use default themes for default configuration of circadian-themes
• Introduce (circadian-setup) to map each time-theme pair to a corresponding timer and load the last overdue theme
• Re-implemented configuration using associated list and timers (thanks to Steve Purcell for pointing me into this direction)