Add MIDI slider device support to deej

[Tthecreator]

Hi all,
I have a device that has 5 sliders running Control Surface (https://github.com/tttapa/Control-Surface)
I wanted it to work with Deej.

I disliked the use of the serial interface, because I use that for other things as well. So it would often give me confusion when doing projects for other microcontrollers. With just a MIDI controller, this is simplified massively for me.

Tested on Linux. Testers for Windows and Mac would be appreciated.

The configuration of the CS device that I used is as follows:

CCPotentiometer potentiometers[] = {
  	{mux.pin(0), {0x07, Channel_1}},    
	{mux.pin(1), {0x07, Channel_2}},     
	{mux.pin(2), {0x07, Channel_3}},
	{mux.pin(3), {0x07, Channel_4}},
	{mux.pin(4), {0x07, Channel_5}}
};

It seems to work pretty well now :).
Please see if we can get this merged so other people can use it as well.