plugin interface

You can build your own plugins by implementing this interface:

``js
export interface ShaderArtPlugin {
name: string;
setup(
hostElement: HTMLElement,
gl: WebGLRenderingContext | WebGL2RenderingContext,
program: WebGLProgram,
canvas: HTMLCanvasElement
): void | Promise;
dispose(): void;
}
``

If the setup method returns a promise, the shader-art component will wait until the promise resolves.