Add support for more markdown flavors and markup languages

[wheelercj]

Currently, Stardown focuses mostly on GitHub Flavored Markdown (GFM), but there are many flavors of markdown and many other markup languages.

Examples of markup languages:

• Comparison of features in various Markdown flavors

• Lightweight markup language - Wikipedia

• List of document markup languages - Wikipedia

• Markup lowdown: 4 markup languages every team should know | Opensource.com

• 18 Best markup languages as of 2024 - Slant

• [x] A setting could be added to let the user choose the markup language of Stardown's output.

• [x] This feature requires many changes to how Stardown converts HTML to other formats, so #105 should be completed first.