Create example sketch(es) for external sensors

[jasonwebb]

I've added a 3-pin interface for connecting to external analog or digital sensors. Usage is pretty simple, but I'd like to provide at least a couple sketches to demonstrate how it could be used.

Sensors to explore include:

• Proximity/distance - thinking of IR rangefinders, but could also work for ultrasonic rangefinders
• Temperature
• Tilt using mechanical sensor or accelerometer for hands-on manipulation
• Sound
• Touch
• Flex or force
• Biometrics
  • Pulse / heart rate
  • Muscle activity
• Environmental
  • Soil moisture
  • pH level
  • Various gas and chemical sensors (LPG, particles, dissolved oxygen, more).

[jasonwebb]

Example for simple digital sensors added - https://github.com/jasonwebb/Jar-of-Fireflies/tree/master/firmware/DigitalSensorExample (commit db0fb9b0f0d7d542fda402aa50b147bec2f344ab).

[jasonwebb]

Example for analog sensors added - https://github.com/jasonwebb/Jar-of-Fireflies/tree/master/firmware/AnalogSensorExample (commit ba47880a6637232c38f7d06e538743938efaf869)

[jasonwebb]

Will break down into multiple issues.