Add ChainableUndefined allowing getattr & getitem (#997)

Add ChainableUndefined allowing getattr & getitem (#997)

* Add ChainableUndefined allowing getattr & getitem

Allows using default values with chains of items or attributes that may
contain undefined values without raising a jinja2.exceptions.UndefinedError.

>>> import jinja2
>>> env = jinja2.Environment(undefined=jinja2.ChainableUndefined)
>>> env.from_string("{{ foo.bar['baz'] | default('val') }}").render()
'val'

* Remove class decorator from ChainableUndefined