tools: make add-on scraper print filenames

Make the tool that generates add-ons from doc/api/addons.markdown print
the names of the files it writes out.  Before this commit, it printed a
rather unhelpful "Done."

PR-URL: https://github.com/nodejs/node/pull/2428


Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: Rod Vagg <rod@vagg.org>