make "navbar_links" conf option to support any arbitrary link.

This change allows "navbar_links" theme option to support any arbitrary link, while maintaining the compatibility of original doc link support. Support of doc links were added in the original commit 01f8cb1.

     'navbar_links': [('Nav1', 'doc1'),
                      ('Nav2', '/link/nav2', 1),
                      ('Nav2', 'http://www.example.com/', 1)]

This is related to my original comment for issue #34

Thanks.