More query builder methods

• .attr_value(Pattern): matches any node with an attribute whose value matches Pattern
• .attr_name(Pattern): matches any node with an attribute whose name matches Pattern
• .class(Pattern): short for .attr("class", Pattern)
• .string(Pattern): matches a text node
• .limit(usize): limits the number of nodes that can be returned
• .recursive(bool): only matches the current node and it's direct children