annotator: Don't add Node constants to global object

As convenient as this may be, it's not great to be reaching out of our
closure. This should no longer be required as of
openannotation/xpath-range@930c8ac.
Bump up tolerance for complexity slightly
annotator: Improve behaviour of Annotator.supported

This commit makes Annotator.supported able to accept an optional scope
(which defaults to the current global object). It can also now return
additional details of what is broken.

This is used by the Unsupported plugin to provide better feedback to
users.