Dynamic preprocessing via builtins and frontmatter variables

[skmagiik]

I have a working fork of this, but wanted to see about the viewpoint of the project before I submit a PR.

The idea is that sometimes you want to dynamically render content in a markdown file, or use the same information in many places and updating that when you need to change it can be tedious.
In my case it's useful to add current date to some markdown files that get live rendered periodically to a display via glow.

So prior to glow rendering, we get variables from the yaml input and between those and some builtins we replace the contents before rendering the glow contents.

Example 1:

---
somevar: "some text"
somenum: 123
---
This is {{somevar}}. Said {{somenum}} times.

Would render:

This is some text. Said 123 times.

Example 2:

Today is {{date}}

Would render:

Today is 2025-09-17

Here is a rendered example: