Implemented M112

[DanNixon]

Checking for M112 after each new command enters the command buffer.
Call get_command() from process_inactivity() to preempt moves and heater.
Response is not instant, however is much faster than pausing print.

[whosawhatsis]

Have you tried telling a machine that all of the endstops are triggered to try to kill any moves in progress?

[DanNixon]

I wouldn't need to, assuming there is room in the buffer serial data would be read from process_inactivity() and a call to kill() stops any moves in progress.

[whosawhatsis]

Needs to be added to the code list in the comments at the top of Marlin_main.cpp.