[gatsby-image]: support for <figcaption> as a child of <Img> component?

[ehowey]

Summary

It would nice to be able to semantically add a caption to Gatsby images, and maybe there already is an issue for this but I couldn't find one.

The syntax could look like this:

        <Img
            Tag="figure"
            fluid={post.featuredImage.asset.fluid}
          >
            <figcaption>
              {post.featuredImage.caption}
            </figcaption>
      </Img>

I would maybe be interested in trying to take this issue on if it isn't already possible?