customize how unread containing maildirs are shown

this introduces 2 faces: mu4e-maildirs-extension-unread-face (inheriting mu4e-unread-face) and mu4e-maildirs-extenion-header-face (inhriting mu4e-header-face), that can be customized to set how unread-containing maildirs are shown in summary.

Signed-off-by: Niv Sardi <xaiki@evilgiggle.com>