Feature request: support image caption

[yzhang-gh]

For now, there is no good way to add a caption to an image in docsify. It would be better to have

1. a way to write a caption, like {% include image.html url="..." description="..." %} in Jekyll (ref)
   or just raw HTML snippet <figcaption>...</figcaption> written by users themselves. Then we need
2. default styles for figcaption, e.g. gray foreground color and smaller font

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Another example: Hugo built-in shortcut

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[nichtich]

See image captions in Pandoc Markdown: An image such as

![Steve Francia](/media/spf13.jpg)

Should be rendered with "Steve Francia" as caption instead of alt text.