Don't create figures from images without caption

[peterbozso]

Right now the library creates a <figure> element even when an image doesn't have caption.

For example this:

Following image is without caption:

![Alt text](https://example.com/image.jpg)

So this won't be a figure.

Will be turned into this:

<p>Following image is without caption:</p>
<figure><img src="https://example.com/image.jpg" alt="Alt text"></figure>
<p>So this won't be a figure.</p>

This is a problem in real life for websites like the "About" page of my blog:

1. HTML
2. Markdown

As can be seen here, the image link is to my profile picture, standing on it's own, which I definitely don't want to turn into a <figure>.

This PR fixes this by not adding a <figure> element when there's no text right after the imagine in Markdown. Please refer to the newly addedd test to see it in action.

[peterbozso]

Also (not to bloat the PR description): thank you @yilei for this nice library! It helps a lot.

[yilei]

Thanks for the PR, I'm glad you find this extension useful!

I used standalone images without captions on my site, i.e. relying on the current behavior.

Instead of changing the default, could you please make this an option to opt-in? The WithImageLink() function is a prior art.

Thanks!

[peterbozso]

Thanks for the feedback @yilei - it's done!

[yilei]

Thanks for making the changes! I made a small tweak to the if statement. Merging.

[yilei]

Also made a new release: https://github.com/mangoumbrella/goldmark-figure/releases/tag/v1.3.0

[peterbozso]

Cool, thanks! :)