code: function injectPaths(paths) {
      u.each(paths, function(path) {
        if (path.inject) {
          // injected css and js sources are always rooted paths
          var src = mkSrc(path);
          if (/\.css$/i.test(src.path)) return opts.injectCss.push(src);
          if (/\.js$|\.es6$|\.jsx$/i.test(src.path)) return opts.injectJs.push(src);
          return opts.log('Don\'t know how to inject', path.path);
        }
      });
    }
docs: add injectable staticPaths to opts.injectCss or opts.injectJs