url: support `path` for url.format

this adds support for a "path" field that overrides
"query", "search", and "pathname" if given.

Fixes: https://github.com/joyent/node/issues/8722
PR-URL: https://github.com/joyent/node/pull/8755
Reviewed-by: Chris Dickinson <christopher.s.dickinson@gmail.com>