Add graphics modes to external monitors.

[roberthoffman04]

- Added support for the new graphics modes to the Advanced Monitor.
- Modified existing "monitor" peripheral to give it its own set of paintutil function to keep compatibility.
- Modified the existing `raycast.lua` program to check for an external monitor first. If it finds one, render to that instead.
- Updated `README.md` to reflect changes

Example Code:

local mon = peripheral.find("monitor")
if not mon then error("No monitor", 0) end

mon.setTextScale(0.5)
mon.setGraphicsMode(2)

local W, H = mon.getSize(2)

mon.drawPixels(0, 0, 0, W, H)

mon.drawPixels(5, 5, 16, 40, 20)

mon.paintutils.drawBox(5, 55, 60, 85, 19)

[sashafiesta]

Hello! This looks really good, but I'm afraid I have to delay merging it for a bit.

The problem isn't the implementation, but the fact that monitors can overwhelm the network thread very quickly.
Adding support for them without first implementing some kind of network throttle would cause major issues for server owners.

I plan to implement the throttling myself, so I should be able to get this merged within the next couple of weeks.