Add support for a per-view custom alt tag

[zackdotcomputer]

Description of changes:
Adds support for a view to have an alt property that will be set as the alt tag on the rendered image. This will override the caption if present and will not render anywhere else.

This also adds a section to the README documenting the caption, alt, and source properties on ViewType.

Related issues (if any):
#369 - This is responding to confusion if using the HTML-as-caption feature without overriding the alt formatter.

Checks:

• Please confirm yarn run lint ran successfully
• Please confirm that only /src and /examples/src are committed
• [if new feature] Please confirm that documentation was added to the README.md

[davwheat]

Looks fab! Thanks for the PR (again)!

[zackdotcomputer]

No problem at all, thanks to you to helping maintain this! I spent all day (eh tbh like 20 minutes) looking at react lightboxes and it's clear this one is the best free one out there, so I'd be remiss to just use it without helping out.

[zackdotcomputer]

(Fwiw I can't merge so I'll just leave this for someone with commit rights to close)

[davwheat]

I can't see the console warn you're talking about. Could you point me to it?

[zackdotcomputer]

Sorry @davwheat - I think my git repos were in a weird state. Force pushed this branch and it's showing up now here.

[zackdotcomputer]

Oops - forgot to hit "re-request review" - lemme know if there's anything else you'd like changed here @davwheat .