Confirm dangerous extruder target temperatures.

Protect the user from typos in setting the extruder temp by confirming
them with the user if they exceed 250.
Refactor and confirm exits while printing.

Presently, one might accidentally terminate a print with a CTR-D while
printing. Now we ask the user to confirm exiting if there is a print
going when they try to exit.
Merge pull request #330 from colah/confirmation

Confirmation on exit