Add spec for Reporter base class
user
```ruby
require 'spec_helper'

describe HamlLint::Reporter do
  let(:reporter) { HamlLint::Reporter.new(double) }

  describe '#display_report' do
    subject { reporter.display_report(double) }

    it 'raises an error' do
      expect { subject }.to raise_error NotImplementedError
    end
  end
end
```
assistant