Parse <img srcset="..."> attribute properly

[Treora]

We currently split by comma, but that is a too simplistic approach. Especially as we use data URLs ourselves, which always contain a comma, we shoot ourselves in the foot when snapshotting a snapshot (related: #13).

The better parsing algorithm has been spec'd by whatwg, and implemented by albell.

We could try fix the srcset npm package while at it (see issue).