Add remove methods

[MPur]

Add has and remove methods to use Config easier
Add tests for method has and method remove
Add .idea to .gitignore for phpstorm's users

[hassankhan]

Hi, thanks for the pull request.

Personally I wouldn't add a remove() method, we've discussed this in the past and don't think it's within this library's scope.

That said, the has() method seems useful, thoughts @noodlehaus @DavidePastore?

[MPur]

I understand. For me, it's like offsetUnset of ArrayAccess.
What do you think If I change remove and i call it forget for example ?

[hassankhan]

To be completely honest, the only reason we have offsetUnset is so we can use ArrayAccess, changing the method name isn't the problem, it's just that we don't want to introduce that functionality at all for simplicity's sake.

Obviously if more people want this, then there's no problem 👍

[DavidePastore]

I agree with @hassankhan, I never had the need of the remove method. has could be really helpful.

[MPur]

Ok, I update and remove remove method.

[hassankhan]

Thanks for the PR!