FormTypePlaceholder visitor new features.

[idybil]

Hi,

I think that the FormTypePlaceholder visitor should be able to:

• Extract from an array which is allowed (ex.: when using a DateTimeType) as you can see in the Symfony documentation : https://symfony.com/doc/current/reference/forms/types/datetime.html#placeholder

• Should skip extraction if the placeholder value is a Boolean, (for now, I rely on the Ignore annotation). As you can see in the Symfony doc, we can put a Boolean for the placeholder https://symfony.com/doc/current/reference/forms/types/choice.html#placeholder

[Nyholm]

Thank you.
I've added #90