Support for resource priority attribute postpone.

This commit adds support for the W3C's draft specification for the
postpone resource priority attribute. Any picture element with postpone
or postpone="true" will not be processed until it is visible on the
screen. Additionally, this commit eliminates the unnecessary loading of
IE8 backup images in browsers that support the full polyfill.

https://ngjira.nationalgeographic.com/browse/NGM-1001

To test:

add `postpone` to all picture elements on page;
ensure that all fallback images use `data-src` and have an empty `src`;
scroll down the page and verify that the pictures are processed when they scroll into view.