Use file's metadata.created as `date` in FrontMatter

I use zola to build my personal blog, it is great, but I thought filling the date section in FrontMatter some how bothering. I have to check the calendar for the date today, and type it in a proper way, or I just create a Date with node and do copy-pasting, but these two ways are still to bothering for me (Yes, I’m a very lazy guy).

IMO, the file’s Metadata::created can used as date for most casual situations when date is not set explicitly, and this feature can be turned off in config file if the user don’t want it.

I already have a demo and I’d love to send a pull request if you think this is a good idea.

Honestly I don’t think this is common enough to warrant being added.