How to make img alt information available in JF2

[barnabywalters]

The current mf2 parsing spec makes alt information available on properties which have been parsed from image elements:

<img class="u-photo" src="/photo.jpg" alt="a photo" />

"photo": {
  "value": "/photo.jpg",
  "alt": "a photo"
}

Currently, this information is lost on conversion from mf2 to jf2.

Potential solutions which come to mind:

• Provide img+alt structures where alt information is available, forcing consumers to check whether every image-based property is a string or img+alt array before consuming (breaks existing code)
• Provide img+alt structures by default on every property which is parsed as an image, with empty alt values if none is found. Breaks existing code but is more consistent.
• Use either the ref(erences) list or a new “alts” list on jf2 objects to provide the alt value, indexed by the image URL. This would not break any existing code afaik

Originally noted at aaronpk/XRay#112