[whatwg] Add crossorigin attribute to SCRIPT tags

[Robert Kieffer]

For reasons documented in
https://bugzilla.mozilla.org/show_bug.cgi?id=696301, I¹d like to propose
that support for the ³crossorigin² attribute be added to SCRIPT tags.

tl;dr - When applied to window.onerror information, the same-origin policy
makes it near-impossible to host script resources on a CDN, while
simultaneously using window.onerror to gather information about JS errors.

(fwiw, this issue also appears in
https://bugs.webkit.org/show_bug.cgi?id=70574)