Feature request: add extra option for specifying description (alt-text)

[roelvangils]

For a while now, Slack has an option to add a description (alt-text) to uploaded images. In Slack itself, it looks like this:

It looks like the API has support for this:

Would you consider adding a command-line option to slackcat to add this alt-text?

GLOBAL OPTIONS:
--alt                       If you're sharing an image, add a description
                            (or alt-text) to help people with visual impairments
                            understand what you’re sharing.
(…)

It would be appreciated! 🙏