Allow XPath queries to extract attributes

It seems [Nokogiri::XML::NodeSet#css](https://github.com/sparklemotion/nokogiri/blob/master/lib/nokogiri/xml/node_set.rb#L94..L118) already supports XPath queries, by parsing the CSS selector to XPath.

Using `Nokogiri::XML::NodeSet#search` makes it more explicit that XPath can be used.