tools: add mock-y js-yaml dependency to doctool

Add a hack js-yaml module to the doctool dependencies that simply
loads the one that’s included with eslint.

This helps avoiding to check in the whole dependency tree into
the core repo.

PR-URL: https://github.com/nodejs/node/pull/6495


Reviewed-By: Robert Jefe Lindstaedt <robert.lindstaedt@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>