_s: Make it easier to target post navigation links.

Removed in 3f4effb when we split the template tag, this will restore
parity with `_s_paging_nav()`.

Props @gatespace for initial patch. Fixes #371.
Merge branch 'master' of https://github.com/Automattic/_s into upstream
Merge branch 'upstream'
Merge branch 'master' into customized