add linear gradient support

[sambauers]

• no build files included in this commit
• should work on shapes, text, charts, and tables
• only a basic demo on shapes included

Example usage:

slide.addShape(pptx.shapes.RECTANGLE, {
 	x: 5.1,
 	y: 6,
 	w: 3.0,
 	h: 1,
 	fill: {
 		type: "linearGradient",
 		stops: [
 			{ position: 0, color: '000000', transparency: 10 },
 			{ position: 100, color: '333333', transparency: 50 },
 		],
 		angle: 45,
 		scaled: 1,
 		rotWithShape: false,
 		tileRect: { t: 0, r: 0.5, b: 0.25, l: 1 },
 		flip: 'xy',
 	},
 });