Support for SVG Icon Stacks

[andydavies]

Would you be up for a PR that adds support for SVG icon stacks - http://simurai.com/blog/2012/04/02/svg-stacks

I think it's a pretty simple change, still need to finish testing but query pattern becomes "svg > symbol[id], svg > svg[id]"

[jaydenseric]

If my very shallow understanding of SVG icon stacks is correct, then an icon could potentially be any element with an id attribute, such as <g>?

My concern is that lots of things have ids, including gradients, masks, groups, and paths. If all implementations have an icon class on icons, the selector can be a lot more robust.

If it can be done reliably a PR is welcome :) The results could be split into two subsections under separate headings, one for symbols and one for an icon stack.