Add toBeIn expectation

[freekmurze]

Q	A
Bug fix?	no
New feature?	yes

This PR adds a new toBeIn expectation.

expect('a')->toBeIn(['a', 'b', 'c',]); // passes
expect('d')->toBeIn(['a', 'b', 'd',]); // doest not pass

[jordanbrauer]

This is nice. It's basically an alias of toContain, right? That's what I've been using to check array contents.