SVGElement dataset

Hello team, 

I would like to propose dataset (https://developer.mozilla.org/en-US/docs/Web/API/HTMLElement.dataset) feature for SVGElement, like HTMLElement.

This feature is available on Safari / Chrome (Webkit), but not on Firefox (not tested on IE, Opera...).
https://bugzilla.mozilla.org/show_bug.cgi?id=921834

Thank you.