Add monochrome output mode to eliminate color artifacting

[fjmilens3]

One of the Cody Computer's users has been using the hires mode for an academic project but NTSC color artifacting is making things difficult. There may be a way to improve the color output to reduce artifacting, but another option is to turn off the color signal and generate a black-and-white signal instead. This behavior would be opt-in with a bit in the video control register. In addition to helping with this particular use case, it might make it easier to port very old (Commodore PET) hires applications to the Cody Computer at some future date.