PID Display customization

[jvetter1]

The current PID display configuration is limited to a few hard-coded options that don't give the user much flexibility.

This issue would address that by:

• Providing complete flexibility over what is displayed
• Allow each line of the display to be assigned independently
• Support display of any temp, temp setpoint, volume, volume setpoint, timer, or fixed value
• Support changing whats displayed dynamically at runtime

Possible extension for this update or another would be to make this configurable from the update utility

[jvetter1]

Reference commit: brewtrollergit/brewtroller@720d87b

[jvetter1]

Pull request reference: brewtrollergit/brewtroller@c8d7ae7