This branch adds some more navigation options to v4 with some flexbox love. It includes a new .nav-justified variant and an example of centered nav links, but only for use in flex mode. Works on inline, tab, and pill variants.
Halfway fixes #17723 (closed). I removed the justified nav since those table styles were super buggy in Safari on browser resize, making for a shit experience for a good chunk of web users.
Also snuck in some heading changes for a clearer table of contents.