support footnotes for references

[simpixelated]

Need to write custom markdown loader to support markdown-it-footnote. See gatsbyjs docs:

To write your own loader or override a Gatsby loader, make a loaders directory at the root of your site that contains directories for custom loaders.

e.g. loaders/markdown-loader/index.js will take precedence over the markdown-loader that Gatsby includes.

See an example of a custom loader in the default starter.

TODO:

• [x] create custom loader
• [x] add markdown-it-footnote to dependencies
• [x] parse footnotes in custom loader
• [ ] update Planning Commission letter with footnotes