Support image ALT text when using photos

[gijswijs]

alt is a required attribute of the <img> element and should be provided in any use of images on indieweb pages from photo posts.

source: https://indieweb.org/alt

[barryf]

You're right. This is important for accessibility and should be supported.

I'll have a think about how to implement this because currently photos are specified as a whitespace-delimited list of URLs.