Content Scheduling

Hi,

I have added a feature so that documents can be scheduled for publishing at a later date. I have a scheduled meta data option, that when set to true (default: false) will not publish any documents who's date attribute is in the future.

This could be used when generating a static site but is probably most useful when used with the regenerateEvery configuration option. Added tests and all are passing.

If accepted into the main branch I will push a documentation update.

Regards,

Jon