[Feature Request] Include page date in the search index

[gamingrobot]

Currently there are multiple include_X config keys in the [search] section.

There are multiple options for adding this:

1. Add include_date to the existing config and update the generator to support it.
2. Have a include list in the config that maps to keys in the page frontmatter (this allows for more than just date to be added to the search index).

[Keats]

Seems ok to me, as long as it's only ISO date format search like it's allowed in the front-matter. I'd go with 1 and maybe 2 for later for the extra section