Add support for pull-up/pull-down resistors

[philipellisis]

The raspberry pi has the ability to add resisters into the mix so that external resisters don't have to be added for something like checking if a switch is closed (which would be an issue for the raspberry pi in the case of 3.3v to ground) please add this functionality to the software so we have less to do on the hardware!

[bennigraf]

In case someone else comes by who needs this: https://github.com/jperkin/node-rpio supports configuring Pull-Up/Pull-Down.

[manix]

+1