skip css files

Sometimes we concat our styles with css files from external plugins, and we need just skip them.

Example: 
```
gulp.src(['../vendors/leaflet/dist/leaflet.css', './src/**/src/**/*.styl'])
                .pipe(stylus())
                .pipe(concat('styles.css'))
                .pipe(minifyCss())
                .pipe(gulp.dest('./public/css/'));
```