Add support for setting custom pinmux

[DhruvaG2000]

Since this project offers a semi-GUI experience inside the console, it would be awesome to visualize all of BB pins in beagle-config and then if the user wants to change something then they can simply use arrow keys to navigate to the desired pin and select using enter and then change it to whatever options there are for that particular pin. This feature request was inspired from the RPi documentation

[SAtacker]

That would be awesome!
Will try it in 0.2