feature: count of subset result

[grigyenn]

I hope this one will not be duplicate :)

expect([{ id: 1, name: 'name 1' }, { id: 2, name: 'name 2' }, { id: 3, name: 'name 1' }])
            .to.containSubset([{ name: 'name 1' }]).ofCount(1); // failed, because there is 2 'name 1'