More defensive yet forgiving rendering of frontmatter

This change tries to do a better job of detecting special characters in the frontmatter and only wraps the title (or individual tags) if they contain special characters.

Also, I tried using the `to_yaml` method (as mentioned in #5), but it was much, much too defensive and wrapped all the strings and date with quotes, and rendered tags as a multiline hyphened list (also not really what I wanted).