Added function Adafruit_7segment::writeColon() - write the buffered colo...

Adds a function to allow the colon state, for a 7-seg display to be changed without having re-write the whole display.
(I made this function a member of the class Adafruit_7segment since it didn't make sense for the for the other classes derived from Adafruit_LEDBackpack - but this meant I needed to make i2c_addr protected rather than private)