Allow users to specify arguments to canvas.getContext().  Arguments are passed as a dictionary stored in options.contextArgs where options is passed to Glsl().  e.g.

var glsl = Glsl({
	canvas: canvas,
	contextArgs: {
    	preserveDrawingBuffer: true
    },
	fragment: fragmentShader,
	...,
});
Merge pull request #12 from kevinb7/master

Allow users to specify arguments to canvas.getContext()