add: plugin paths now found relative to cwd
add: plugin paths now found relative to cwd

This allows css-moduleify to be installed in a different directory than
the post-css plugins and still allow them to be found.
bkvfoundry/css-modulesify,joeybaker/css-modulesify,LoganBarnett/css-modulesify,spion/css-modulesify,wenbing/css-modulesify,gilesvangruisen/css-modulesify,css-modules/css-modulesify