Implement the util.inspect.custom symbol

[kriskowal]

nodejs/node#15549

Ideally, Q should add support for util.inspect.custom so that logged promises get the right output and forward the seen set. This would precede the inspect method, which we obviously cannot remove since that would break dependees.

This poses a challenge since Q v1 does not depend on require or, if require exists, that it is even provided by Node.js, so getting the symbol in a cross-compatible way will be tricky.