Add colorType: 'default' for pixel prints

[tresf]

A question came in today:

[...] why is QZ Tray using it's own colour settings & not adhering to the windows print queue?

This is a very good question. I've done a code review, and I believe colorType: null will allow QZ Tray to honor the printer's default preferences, but this is not available in the demo (sample.html) nor is it documented in the API. We should have a more intuitive value to honor the printer's default colorType.