Add support for include/exclude options about cosmere

[mihaeu]

I'm closing this issue for now, because it's quite old at this point. Sorry for not getting to it earlier. If this is still relevant let me know.

The reason why I initially decided against frontmatter was because it makes the markdown files harder to read and they just become pre-processed source material and aren't fun to read in the editor anymore (which is relevant for code information).

That being said, there's no reason to allow this in addition to the list in the config for those who prefer to publish this way. Let me know if there's still interest.