_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 remote-tracking branch 'upstream/master' into ja

* upstream/master:
  _s: Make it easier to target post navigation links.
Merge remote-tracking branch 'upstream/ja'

* upstream/ja:
  _s: Make it easier to target post navigation links.