More Graphics Additions

[judy-zz]

• Basic background colors
• Just rgb values for now. I have some ideas for doing an easy but powerful color system. RGB, HSL, HSV, named colors, color manipulation, and so on. I'd love to do all of that at once, in the future.
• Line and line width
• Alpha blending.

This is going more smoothly, now that I've locally loaded a complete sketch from Zajal 0.3.5, and I'm going through each missing command one at a time. I can go back to doing a separate branch and pull request for each new command added, but I believe this is faster. Let me know what you want.

[nasser]

I left out your implementation of Graphics#background until we can nail down a coherent strategy for the 'state' setting functions that plays nicely with live coding. Aces all around other than that!