Improve page-navigation boundary detection

[dragonstyle]

• Use _metadata.yml to enable page-navigation for a section of a site
• Don't enable on the next or previous sections
• The page navigation shouldn't continue to next and previous sections b/c they don't support page-navigation.

I am experimenting with page-navigation: true within a sub-directory _metadata.yml file. The pages within this directory are creating navigation next/prev out of the directory. I can modify the sidebar navigation with --- separators, but that feels excessive, because now I’ve got separators as the first and last items in this nav sub-tree. Are there other ways to accomplish this? I’m happy to create a small example if that would be helpful. (edited) 

[dragonstyle]

This is pretty tricky as we currently don't know whether the next / subsequent sections have page-navigation enabled or not, unfortunately. Moving to 1.5