Add helpers for modifying a document

[brandonchinn178]

Add a helper using KDL Query Language (#10) to modify nodes in a document:

-- foo { inner { bar { baz } } }
-- =>
-- foo { inner { bar { baz2 } } }
doc.modifyNodesMatching ("foo" KDL.>> "bar" KDL.> "baz") $ \node -> node{name = "baz2"}