Allow the specification of alt-text on images when using CLI.

[PeterGrace]

This PR adds the ability to specify alt-text on images to help give descriptive values to them. It is written so that the alt-text input is a one-to-one map of strings based on the image filename input.

Let me know what I need to do to help get this approved and merged! I'm not sure what level of documentation is required since I assume bsky-cli is a proof-of-implementation tool, however it works just fine for me for what I need it to do so figured I'd improve it!

[sugyan]

Thank you for the pull-request!
Please check the format first with cargo fmt-no-gen --check.

It is curious that images is plural and alt_text is singular, even though both are Vec. Shall we change it to image to align them?

I see that it also takes into account cases where the number of images does not match, such as --image img1.png --alt-text alt1 --image img2.png. If there is no corresponding idx alt_text, the file name is still used by default. (In this example, the alt text for the second image is “img2.png”.)
Conversely, I would like to consider the case where we want to specify only the second. If I do --image img1.png --alt-text “” --image img2.png -alt-text alt2, I want the alt of the first image to be “img1.png “.

For example, how about the following?

let alt = match args.alt_text.get(idx).filter(|s| !s.is_empty()) {
    ...
};

[sugyan]

It is OK to change this to image.

[sugyan]

I suggest adding .filter(|s| !s.is_empty()) or something like that, since an empty string will be assumed to be unspecified and the default filename will be used.