WIP: Try rubocop

Rubocop was discussed a bit a while back.  I thought I'd give it a shot.  This is a WIP trying to figure out what checks to enable/disable and whether or not we want to use it.

Feedback wanted.
What do you think about this style:

```ruby
def backtrace_formatter
  @backtrace_formatter ||= begin
    if defined?(::RSpec.configuration.backtrace_formatter)
      ::RSpec.configuration.backtrace_formatter
    else
      NullBacktraceFormatter
    end
  end
end
```