Add "ifContains" method for Colada\Collection, with tests

Hello.

I really want something like this:
```php
public function saveUser($newUser)
{
    collection($this->getAllUsers())->mapBy(x()->getEmail())->ifContains($newUser->getEmail(), function() {
        throw new DuplicateEmailException();
    });

    $this->save($newUser);
}
```
instead of this:
```php
public function saveUser($newUser)
{
    if (collection($this->getAllUsers())->mapBy(x()->getEmail())->contains($newUser->getEmail())) {
        throw new DuplicateEmailException();
    }

    $this->save($newUser);
}
```