Support YAML frontmatter

[foxfriends]

Markdown documents sometimes have YAML frontmatter at the start, which does not render nicely when piped into paper. In particular, the output of gh repo view, for example, includes this frontmatter, so gh repo view | paper does not look great at the top.

The frontmatter should be parsed and removed. It could be shown off the page, or in a table on the page, if desired (maybe put it behind a flag?).

[foxfriends]

as of the recent change, pulldown cmark supports parsing this. i'll just have to render it properly