Add linkable headings like in GitHub markdown flavour

[iamandrewluca]

Would be nice to have links to every heading (or at least h1 and h2) in a blog post page, so we can reference them easier.

Ex: https://github.com/kentcdodds/kentcdodds.com#contributors

Notes:

• https://github.com/gatsbyjs/gatsby/tree/master/packages/gatsby-plugin-mdx#mdxprovider
• https://github.com/kentcdodds/kentcdodds.com/blob/master/src/components/mdx/index.js#L12
• https://github.com/kentcdodds/kentcdodds.com/blob/master/src/components/mdx/title.js

[kentcdodds]

Agreed. I'm pretty sure all it'll take is a gatsby/remark plugin. Want to investigate that and make a PR?

[iamandrewluca]

Don't have to much time right now, working on other OSS projects.
This is a easy one. I'll come back after some time and check this, if will not be done already.

[iamandrewluca]

https://www.gatsbyjs.org/packages/gatsby-remark-autolink-headers/

[iamandrewluca]

It seems this was implemented.