cache icons

[tvdr]

Hey, first of all, it's a great component.

While debugging ,i've noticed, that every sorting fires a network request - which some would consider as suboptimal :)

This PR offers one way to tackle this, fetching the svg once on init and inserting it hidden into the dom.

[indgy]

Thanks for pointing this out, it's definitely not ideal to be issuing spurious network requests.

I had thought that by using an SVG sprite for the icons that the sprite would be loaded once then each icon would be fetched from the already loaded sprite. I'll check why this is not the case first as I'd rather avoid adding more code if possible, the svg sprite should be cached by the browser so using native functionality is a cheap easy win.

I'll reply when I've tested it more, thanks.