[gatsby-plugin-mdx] Support shortcodes on feed HTML

[rogermparent]

Summary

The ability to override the React wrapper used to generate the html field in GraphQL.

Basic example

In the plugin definition's options, allow for the field staticHtmlWrapper. If provided, this string will replace value of this line. I would imagine when overriding defaults with this field, it should be on the user to use require.resolve and ensure the component is present.

Motivation

This enables shortcodes (global components) to be used in RSS feeds pretty easily, and otherwise users get a ton of warnings about undefined components. Being able to use different, more RSS friendly versions of the components would be nice as well.