Some more W3C syntax superpowers

[doasync]

Add these W3C syntax superpowers (from cssnext):

---

https://github.com/SuperOl3g/postcss-image-set-polyfill
(W3C)

.foo {
    background-image: image-set(url(img/test.png) 1x,
                                url(img/test-2x.png) 2x,
                                url(my-img-print.png) 600dpi);
}

---

https://github.com/postcss/postcss-color-gray
(W3C)

.foo {
  color: gray(80, 30%);
}

---

https://github.com/postcss/postcss-color-hex-alpha
(W3C)

.foo {
  color: #0000ffcc;
}

---

[jonathantneal]

This is all possible in PreCSS v3 :)