Provide alternative way to determining whether a page ends up in top navigation

[wssbck]

To determine whether a page is being shown in top navigation, do not rely on certain tags present in pages. Instead, allow for an explicit setting in page properties.

[ashawley]

Instead of introducing a new config variable in the front matter, pixyll should try to be more consistent with other jekyll themes and use the header_pages variable in the site config. Here's the config of the default theme https:/jekyll/minima/blob/master/_config.yml that is used with jekyll new site command. I'm not sure if it's used in others, though.

I've made this change for pixyll in #334, which is a PR for making the theme in to a gem. I'm happy to submit that change as a separate PR from the gem one. Since this is a breaking change, I'd say that it's worth making the change to be consistent with other themes, than not.

[wssbck]

@ashawley Didn't know about that, thanks for suggesting. If you have nothing against it, I could replicate your change on this PR. However if you would prefer to open your own, please do, of course

[ashawley]

Sure, I'll work on pulling that change out to a new PR

[johno]

Thanks for opening up this pr, we've incorporated your proposed in #342 so I'll go ahead and close this now 💟