Filter out hidden posts

[bkoprucu]

Jekyll doesn't list the posts with a front matter variable hidden: true or hidden: 1 in the paginator / posts listing page.

This change applies the same behavior to the generated feed.

[guilhermesimoes]

Would be great to have this merged

[parkr]

Hey, this would match the jekyll-paginate gem. There's also published: false. Jekyll's docs don't really describe when to use one vs the other. @mattr- Should we make this align with the pagination plugin?

For now, please update the README here to indicate that posts marked hidden are removed from the feed.

[mattr-]

We should match the paginate gem so that things are consistent. I admin that I don't remember why we have published and hidden properties that seem to do the same thing. Something to investigate later.

[iBug]

My understanding is that published allows a post to stay in the work tree while not being built at all (i.e. as if it doesn't exist, unless with --unpublished). hidden allows a post to be built while omitted from listings (i.e. pagination). This PR updates feeds to match the behavior of pagination, as both are generally considered listings.