Add LINQ-style selector methods

Issue #11 resolved
kevin.w.berridge@gmail.com
repo owner created an issue

Single, SingleOrDefault

Where {{{ <root> <child1 name="a">value1</child1> <child1 name="b">value2</child1> </root>

x.root.child1.single("name", "a") == "value1" x.root.child1.where("name", "a") == ["value1"] }}}