Support native loading attribute

[danielck]

Currently Chromium supports the loading attribute on img and iframe elements, which can speed up initial page load considerably especially on mobile.

It's still a draft in the WHATWG spec and not in the W3C spec, as far as I know.

Since adding the native attribute would immediately benefit 62% of users (according to caniuse.com) I suggest adding support to the DP image helper by directly supporting adding the loading attribute.

Spec: whatwg/html#3752
Firefox bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1542784
Polyfill: https://github.com/mfranzke/loading-attribute-polyfill