Auto-format YAML frontmatter

[jlevy]

As a convenience, Flowmark should work on a Markdown file with YAML frontmatter, and then auto-format the YAML. This would also catch lots of syntax errors, normalize quote usage in YAML, etc., which are ongoing pain points for Markdown files with frontmatter.

Also as a convenience this should work on plain YAML the same way (need to auto-detect somehow even if there are no --- markers however).

Settings should be 2-space indents, and other sane defaults. Probably insist on quotes for most strings too, if possible. (Things like @ inside strings breaking YAML is a common pain point for authors.)