Support Markdown rendering

[khalidabuhakmeh]

Allow folks to take a markdown file and render it to the console.

https://github.com/charmbracelet/glow

[mcon]

I'm going to have a shot at this, looks like it's not too tricky to use https://github.com/xoofx/markdig (I'll be working here: https://github.com/mcon/spectre.console/tree/markdown - should have something of decent quality shortly)

[mcon]

PR here if anyone's interested: #204