emacs: add default value to notmuch-search-line-faces

Add default value to notmuch-search-line-faces to show "unread"
messages in bold, and "flagged" messages in blue, to have some visual
indication of important messages in search results. This should be
helpful for new users.

"unread" tag is quite obvious, and handled specially both in the lib
and emacs ui. "flagged" is synced to maildir F flag in the lib. If one
syncs the maildir to IMAP, this also translates to corresponding IMAP
flag. (This is "starred" in GMail and Android.)

Signed-off-by: Jani Nikula <jani@nikula.org>