Allow Gcodes to be entered while printing.

[jgbrown54]

It would come in handy sometimes to be able to enter Gcodes while printing. For example: M412 to enable or disable the Runout sensor. Since I use a Raspberry Pi as an interface, I don't have an issue because I can enter Gcodes whenever I want. But it seems that most people print from an SD card so they don't have to leave their PC on and connected for days.