Handle date frontmatter better

[tdreyno]

New approach:

Find the most descriptive date from the path:

• /blog/mypost.html (no data)
• /blog/2012/mypost.html (year)
• /blog/2012/12/mypost.html (month)
• /blog/2012/12/31/mypost.html (day)

Then read frontmatter.

If we have only year from the path, set frontmatter to full date. If we have only year/month from the path, set frontmatter to full date.

If we have year/month/date and the frontmatter matches: remove frontmatter. If we have year/month/date and the frontmatter matches, but it more precise (HH:MM:SS), keep frontmatter.