Add component to do figure & figcaption for images within MDX

[richwklein]

It would be nice to show a figure and figcaption for images in posts. We can do something with the img tag via shortcodes. See what we did with the <MDXLink> component. When the image has a title attribute we use that as the <figcaption> and we wrap the image in a <figure>.

We would need to figure out how that interacts with the remark-image plugin that is already being used. https://www.gatsbyjs.com/plugins/gatsby-plugin-mdx/?=prism#gatsby-remark--plugins

Here is an npm module that can give us inspiration https://www.npmjs.com/package/@bradleyburgess/img-figcaptions or https://github.com/leonardodino/gatsby-remark-figure-caption/tree/master