Feature Request: Allow setting alt text for card images

[lphillips]

Feature Description

Currently, the alt text for images set in a card uses the title string for the card. While this is a reasonable default, it would be nice to be able to provide other alt text as a parameter to the card shortcode. The alt text could be optional, defaulting back to the title if the alt text isn't provided.

Problem/Solution

Some site designs may benefit from the ability to provide different alt text for some cards.

Alternatives Considered

I've overridden card.html to add this functionality locally. It's simple to add, and might be useful to more people.