tools: increase lint coverage

Extend linting to tools/license2rtf.js and any other JS that gets added
to the `tools` directory by default.

This incidentally simplifies lint invocation.

PR-URL: https://github.com/nodejs/node/pull/7647


Reviewed-By: Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>