Support react component rendering

[zsherman]

Some inspiration:

• Using React components in Markdown source gatsbyjs/gatsby#312 (comment)
• https://www.npmjs.com/package/reactdown
• New gatsby-remark-embed-snippet plug-in gatsbyjs/gatsby#3012
• http://md-to-react.surge.sh/
• https://github.com/fazouane-marouane/remark-jsx/tree/master/packages/remark-custom-element-to-hast

[yjose]

a good approach here https://github.com/probablyup/markdown-to-jsx

[mrpotatoes]

@yjose It uses dangerouslySetInnerHTML though. Something like remark does not which would be a better option imo.