let user customize "on_failure" and "on_success"

[adelevie]

it "is accessible" do
  on_failure = Proc.new do
    return false
  end
  on_success = Proc.new do
    return true
  end

  def site_is_accessible?(path)
    (Ra11y::Site.new(path, on_failure: on_failure, on_success: on_success).run
  end

  expect(site_is_accessible?("./_site")).to eql(true)
end