Hi, thanks for this awesome lib!
One question: it seems to be impossible to set a custom return in case the search fails.

E.g.

not_found = object()
jmespath.search('foo.aaa', {'foo': {'bar': 'baz'}})  # I need `not_found`, not `None`

Is this by design or are you open to supporting that?
I see this was discussed at #113
But the proposed solution of contains isn't great, as it requires 2 searches.