Text-stroke and Paint-order support

[spacedevin]

This ads support for -webkit-text-stroke, -webkit-text-stroke-width, -webkit-text-stroke-color, and paint-order.

Note that even through these are webkit prefixes, Firefox still uses them.

paint-order is required in order to get strokes to render as an outline of the text and not in the middle.

paint-order does not currently work fully in chrome, as chrome reads the value of the property, but does not apply it in the dom. Firefox and Safari work just fine.

See tests/text/stroke.html for an example

---

Fixes #759 and #1108

[ivictbor]

Thanks a lot @arzynik , I've published your branch to npm: https://www.npmjs.com/package/html2canvas-stroke , it unblocked https://github.com/devforth/painterro text-stroke feature (though html2canvas-stroke is very updated, probably someone has time capacity to merge a new changes or needs this, poke me)