allow specifying the default color for new pixls.

js/pixl/public/index.html

		pixl.window = {w: window.innerWidth, h: window.innerHeight};
		pixl.world = {};
		pixl.pos = {x: 0, y: 0};
		pixl.color = "black";
		pixl.size = 20;
		pixl.stage.width = pixl.window.w;
		}
		pixl.draw_pixl = function(pos, color, options) {
			var color = color || "black";
			var color = color || pixl.color;
			pixl.ctx.fillStyle = color;
			var screen_pos = pixl.to_screen(pos);
			pixl.ctx.fillRect(screen_pos.x, screen_pos.y, pixl.size, pixl.size);
			var w2 = Math.round(pixl.window.w / pixl.size / 2);
			var h2 = Math.round(pixl.window.h / pixl.size / 2);
			pixl.ctx.fillStyle = "black";
			for (var x = pixl.pos.x - w2; x < pixl.pos.x + w2; x++) {
				for (var y = pixl.pos.y - h2; y < pixl.pos.y + h2; y++) {
					var pt = pixl.world[[x, y]];
			if (pixl.drag.current === undefined) {
				var ev = polyfill.movement(ev);
				var world_pos = pixl.to_world({x: ev.clientX, y: ev.clientY});
				pixl.draw_pixl(world_pos, pixl.at(world_pos) ? "white" : "black");
				pixl.draw_pixl(world_pos, pixl.at(world_pos) ? "white" : pixl.color);
			} else {
				console.error("drag not implemented");
			}