Add settings to customize page grouping

[toddhgardner]

My initial thought is that we can set up the grouping in a pretty easy way in the settings - for instance, be able to set up what custom post types should be called, or specific URLs. I can see something like this kind of thing ACF uses...

For instance

Group name [text input] when [post type ⌄] [equals ⌄] [case study ⌄]