Here's a brief explanation of how to use this strange little widget. Full documentation of the libraries used in this project can be found in the above links.
The "template" field takes a nunjucks template. Basically normal HTML with inserted values in brackets:
<h1> Hello {{ name }}! <h1>
Currently the only way define keys and set values is through YAML frontmatter in the markdown:
---
favoriteColour: purple
---
Lorem ipsum...
Now "favoriteColour" can be used in the template between curly braces. One value can be used multiple times, which can be useful for things like titles which may need to be in the metadata of some HTML, but also rendered in the body. Note "{{content}}" is always a valid property name and inserts the rendered markdown that follows the YAML frontmatter (this means that if you just want to render the markdown, you can write "{{content}}" with no other markup)