Add option for MultipleExpectation to special case aggregate_failures

[backus]

As @JonRowe mentioned in #228, the general guideline is to have "one business rule expectation" per spec. Sometimes this isn't always possible with a single expect(...). It would be nice to support something like

aggregate_failures("verifying response") do
  expect(response.status).to eq(200)
  expect(response.headers).to include("Content-Type" => "text/plain")
  expect(response.body).to include("Success")
end

being treated as one expectation

[JonRowe]

Great way to represent it.

[Darhazer]

I think there should be an option for MaxAggregatedExpectations as even with aggregate_failures 10 expectations might be too much.

Also an option may determine if RSpec/MultipleExpectation recommends using aggregate_failures when there is more then whatever the maximum expectations are set. With such recommendation, it is possible to implement an autocorrect as well.

[backus]

This was implemented in #329