Altered date formatting to be trigger also by key

When importing from jekyll / octopress , the date string in the YAML front matter can also be formatted as a datetime string, including the time.

When this occurs, metalsmith supplies the date as a string.

I added an override that if the key is 'date' and it's not an instance of date, try to parse it as a date anyway.

Also added test.

Cheers and thanks for a lovely plugin :octocat: !