Add a Rubocop autoconfig

This excludes all the existing problems reported by Rubocop so that we don't get depressed by seeing every single module fail the check.

The goal is to at least prevent new classes of Rubocop errors from cropping up, and provide a TODO list of fixes to make across our giant code base.

Incidentally, this method of starting out with Rubocop is suggested in Rubocop's [README.md](https://github.com/bbatsov/rubocop#automatically-generated-configuration).

CC @jhart-r7