its with arguments

It is very common to write specs for testing on multiple arguments:

``` ruby
it "should be 1" do
  subject.check(1).should == 1
end

it "should be 22" do
  subject.check(2).should == 2
end

# etc
```

It would be way MUCH easier to write:

``` ruby
its(:check, 1) { should == 1 }
its(:check, 2) { should == 22 }
```

It makes it absolutely clear what's happening with 2 lines of code instead of 6.
