util: allow symbol-based custom inspection methods

Add a `util.inspect.custom` Symbol which can be used to customize
`util.inspect()` output. Providing `obj[util.inspect.custom]`
works like providing `obj.inspect`, except that the former allows
avoiding name clashes with other `inspect()` methods.

Fixes: https://github.com/nodejs/node/issues/8071
PR-URL: https://github.com/nodejs/node/pull/8174


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>