Callback functions

Attaching a function example:

```c++ This example demonstrates how to attach a function to some line. The callback function will change a variable and write its value to a pin. It will be attached to led_line under the number 1.

... const byte ledPin = 3; We need a variable that holds the state. byte ledState = 0; LiquidLine led_line(0, 0, "LED: ", ledState);

/* Callback functions for changing some state need to change the variable that is printed on the display and in this case write it to the appropriate pin.