Add options to hide notes from RSS feed and site map

[russellgordon]

There are currently options to hide pages from the file tree or graph views.

For potentially similar reasons, it would be helpful to hide pages from the RSS feed or site map.

I've already hacked around in feed.njk and sitemap.njk to make this change (basing the conditional on the frontmatter tags --dg-hide and --dg-hide-in-graph). Since I don't use those features in my own website, I can re-purpose values found in those frontmatter tags to suit this purpose (hiding pages from the RSS feed and site map).

I'd like to do this change properly though, and I'd be happy to make the code changes myself and contribute a pull request – the only thing I'm not certain about is how to add new frontmatter tags.

If anyone can provide guidance on that front, I'll get to work and create a pull request with the necessary changes.