Would it be better for rules to register themselves with this class, so that this hard coded list isn't necessary here?

It could be something like:

```ruby
module Yardstick
  class Document

    @enabled_rules = []

    def self.enable_rule(rule_class)
      @enabled_rules << rule_class
    end

    def self.measure(docstring, config)
      document = new(docstring)
      MeasurementSet.new(@enabled_rules.map { |rule_class|
        Measurement.new(document, rule_class.new(document, config.options(rule_class)))
      })
    end
  end
end
```

Then each rule could register itself independently.