more color updates: add a "default" syntax that takes no extensions for

more color updates: add a "default" syntax that takes no extensions for
those files that don't match any other syntax's extensions (ported from
Brand Huntsman's old patch, but modified to work with a syntax named
"default" instead of requiring a "default-syntax" keyword, and to fit
into the existing color architecture)

git-svn-id: svn://svn.savannah.gnu.org/nano/trunk/nano@2959 35c25a1d-7b9e-4130-9fde-d3aeb78583b8