Add `a_hash_including` alias.

The other aliases didn't really fit for a hash but a hash
is one of the main types of objects used with the `include`
matcher.

I'm working on a blog post about composable matchers and of the examples I'm using would really benefit from this alias, and I think it's worth having in general.  The example, FWIW:

```` ruby
class BackgroundWorker
  attr_reader :queue

  def initialize
    @queue = queue
  end

  def enqueue(job_data)
    queue << job_data.merge(:enqueued_at => Time.now)
  end
end

describe BackgroundWorker do
  it 'puts enqueued jobs onto the queue' do
    worker = BackgroundWorker.new
    worker.enqueue(:klass => "SomeClass", :id => 37)

    expect(worker.queue).to match [a_hash_including(:klass => "SomeClass", :id => 37)]
  end
end
```

/cc @xaviershay