console: rename argument of time and timeEnd

Name it timerName instead of label. It is clearer that way and matches
the description in the doc. It is also how it's named in MDN.

PR-URL: https://github.com/nodejs/node/pull/3166


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>