Introduce --link-module to ./configure

- Allows specifying a _third_party_main outside of the node repository
- Allows embedders to create custom builtin modules outside of node's
  repository

PR-URL: https://github.com/nodejs/node/pull/2497


Reviewed-By: fishrock123 - Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: jasnell - James M Snell <jasnell@gmail.com>
Reviewed-By: evanlucas - Evan Lucas <evanlucas@me.com>
Reviewed-By: cjihrig - Colin Ihrig <cjihrig@gmail.com>