MDX

Is your feature request related to a problem? Please describe.
I don't think Theme UI should be opinionated on which SVGs should be used for the Radio & CheckBox components

Describe the solution you'd like
Probably an additional prop called customIcon maybe... based on which we could conditionally render the passed SVG icon otherwise we use the default icons?

**Describe alter

Things like Optional Chaining should be transpiled for e.g. node 12 compatibility: https://esbuild.github.io/content-types/#javascript. We should choose es2016 for all packages.

Describe the bug

GitHub provides a few options when configuring a new repo like: license, .gitignore file, a README.md, and the Rennovate plugin. If you want to install any of these by default, which I usually do, then you get an error about the remote having newer content.

To Reproduce
Steps to reproduce the behavior:

1. Create a repo with a readme during the script
2. See the er

Would it be possible to include excerpts from markdown files in the ‘Referred in’ section?

Currently, if the mention comes from notes originating from Roam, the parent block with the page mention is shown as a bullet point unter the page title.
But for local markdown files, only the page title is given.

<img width="592" alt="Screenshot 2020-06-19 at 17 50 49" src="https://user-images.github