Default date to use filesystem info

By default, pelican tries to fill in certain metadata automatically. This means that when working on a draft, for example, you can omit metadata like the publication date until you know when you want to publish it. However, the out-of-the-box setting for DEFAULT_DATE in pelican is None which means that it doesn't fill in the date for you.

Switching DEFAULT_DATE to 'fs' lets me omit date and use filesystem modification data for the date if I haven't specified one. This lets me work on drafts without entering the date metadata.

Without doing this, make html will fail for articles (but not pages) that do not specify a date.