Support `<img srcset>`

[twm]

Pass through the srcset attribute from feed content.

This implies some measure of support for the sizes attribute, which is required when using NNNNw specifiers. It can be used for art direction, but I don't know how commonly that's actually done. It may be possible to coerce the value to auto.

Prerequisite: teach feedparser how to resolve relative URIs within the srcset attribute.

Motivation: /feed/666/ has broken src attributes (such as in /article/281222/).

[twm]

After looking at the sizes spec I think that the pragmatic thing to do is to strip srcset if it contains w descriptors.

[twm]

#1292 is in, and kurtmckee/feedparser#509 is pending upstream.