Adding a new way to declare lists. Now user can combine lists. Updating R...

I've created a patch (with tests and README updates) that permits to create fixtures combining lists, like this:
```yaml
Nelmio\Entity\User:
    user{alice,bob}_type{a,b}
        username: <current("0")>
        fullname: <current("0")>
        userType: <current("1")>
        birthDate: 1980-10-10
        email: <current("0")>@example.org
        favoriteNumber: 42
```

This will create 4 uses: alice_a, alice_b, bob_a, bob_b.
I've also changed the way that <current()> provider works, so we can pass a parameter telling from which list we want the value.