Add containJson argonaut matcher

# {a: 1, b: 2, c: { d: 3 } } should containJson {a: 1, c: {d: 3}}

{a: 1, b: 2, c: { d: 3 } } should beIdenticalTo {a: 1, c: {d: 3}} ignoringNot("/a, /c/d")
where the paths are derived from the expectation.
