Add checks that drafts are not added to the sitemap.xml

[DeonHeyns]

If you have any drafts in your _posts folder they will be added to the sitemap and potentially indexed by the bots

[benbalter]

Any way we could use the new where filter? Perhaps we should implement a where_not filter or similar to support the negative case?

[parkr]

Good catch! A couple things:

1. Drafts go in the _drafts directory. Unpublished posts (published: false) != drafts
2. Drafts will be added to site.posts only if show_drafts: true is set in the _config.yml or if the --drafts flag is filtered. Usually neither is set outside local development

Definitely agree that where could be used more judiciously. As-is, this PR is unmergable, however, as it doesn't represent drafts in Jekyll.

[kleinfreund]

Note that posts with published: false are not included in the sitemap.

[DeonHeyns]

Thanks for the feedback, was not aware of the published flag.