Support attributes in references

[finestructure]

![An "Arena Gala" on iPad][image-1]

[image-1]:	https://f000.backblazeb2.com/file/finestructure-public-images/posts/arena-gala-on-ipad.png width=954

renders as

<img src=​"https:​/​/​f000.backblazeb2.com/​file/​finestructure-public-images/​posts/​arena-gala-on-ipad.png width=954" alt=​"An " arena gala" on ipad">​

[finestructure]

Edit: the whitespace is not the issue, that fact that it's a reference link it.

NB: This might include several separate problem: the whitespace in the caption, the quotes, and the fact that the "width=954" is tacked on to the end of the footnote instead of being inline with the link.

[finestructure]

It doesn't look like there's a sane way to fix this. Inside the modifier handler imageAttributes() we've only got

(lldb) po html
"<img src=\"https://foo.com/image.png width=954\" alt=\"An \"Arena Gala\" on iPad\"/>"

(lldb) po markdown
"![An \"Arena Gala\" on iPad][image-1]"

So we can't reparse the markdown because it doesn't include the reference. And parsing the html instead is a bad idea(tm).