Review the markdown-it plugins and see which will be useful

[davep]

There are some handy plugins available to the underlying Markdown parser library we use, so do a review of them and see about adding any that would serve Frogmouth well.

For example, I can see a plugin that could help with #15.

This might actually be more of an issue for Textual itself, as perhaps some of these plugins will require further widget choices when rendering the Markdown widget.

[davep]

See also #46.

[davep]

This can progress now that Now that Textualize/textual#2803 is part of Textual.