Add alias for contain and include

[lo1tuma]

It would be nice to have a contains and includes alias for contain and include.
So you can write the following assertion:

expect(someObject).to.have.property('someProperty').that.contains('anyString');

[keithamus]

Thanks for the issue @lo1tuma, seems like a sensible and straightforward request. Here's where include and contain are defined - so it should be trivial to simply copy those two lines to provide includes and contains. If you have the time to make a pull request then you'll get the credit and end up in our hall of amazing contributors.

[lo1tuma]

@keithamus thanks, working on it.