VIKI has buzzer. Also allow PANELOLU2 to be configured without changing library header file.

Hi Sam,
Here's a couple of changes you might consider. Firstly the Panucatt VIKI also has a buzzer so it is not Panelolu2 only (the early prototypes didn't). 
Also as this is designed to be a library file, it is less than optimal to have to change the library header file to get it to operate in "PANELOLU2" mode. 
So the other change I made is to add an argument to setMCPType to allow you to configure the button bits which are used. I have also maintained the PANELOLU2 define current behavior to avoid any merge problems. 
This does have the minor cost of one each byte and the additional inclusion of readRegister and setRegister for non Panelolu2 panels but I think avoiding the need to have to edit the library headers for the PANELOLU2 is worth it.
Anyway see what you think. 
I have a Marlin pull request open at the moment which adds both Panelolu2 and VIKI support ( https://github.com/ErikZalm/Marlin/pull/407 ). Once an updated LiquidTWI2 library is released then I can enable the buzzer for the VIKI and not require that the library header is changed for the PANELOLU2.