tools: improve js linter

This commit switches from the eslint command-line tool to a custom
tool that uses eslint programmatically in order to perform linting
in parallel and to display linting results incrementally instead of
buffering them until the end.

Fixes: https://github.com/nodejs/node/issues/5596
PR-URL: https://github.com/nodejs/node/pull/5638


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>