Added a new combined mode example

This is a modification of all-in-one.py. It adds another mode where all EnviroPlus and PMS5003 sensor readings are combined on one screen.

Each variable that is displayed can have custom warning limits assigned which change the color of the text according to a predefined RGB palette. It allows for a quick glance of all sensor readings at once in order to jugde if everything is OK in the air or to quickly pinpoint a sensor reading that requires attention.

In addition, the new combined mode saves each reading as soon as it is received for graphing later. As in all-in-one.py, moving your finger close to the proximity sensor switches the mode.