Implement implicit skip behavior

[haihige]

By default, skipping is explicit, meaning that all pages are included in the RSS feed, unless explicitly set to be skipped. This commit allows the user to flip this behavior, so that skipping is implicit: all pages are skipped, unless set to not be skipped.

This is useful for sites with many pages that do not need to be in the RSS feed, where it becomes inconvenient to add the skip frontmatter header to each page.