Mirror how docs are checked in other repos.

First step towards syncing with `rspec-support`'s tooling.
Add Rubocop style standards to build.

- Move custom excludes and non-ideal settings to .rubocop_todo.yml
- Inherit the todo file from the main rubocop config
- Add generic description about matcher module.

  We should consider moving the individual generator methods into this
  module. This will allow us to properly document the matchers as in
  rspec-expectations. We can also apply autoload if necessary at that
  point.
Merge pull request #1118 from rspec/add-rubocop

Use Rubocop