Allow static class vd-menu to be dynamic

[Shakarhaba]

Allow to edit vd-menu__{whatever} to be {somethingelse}__{whatever}
or allow to attach another class to every component please

[Mathieu Stanowski]

Hi @Shakarhaba.

Added a new prop : content-class.
It applies a custom class to datepicker content. You may try this, and let me know if it's ok.

It would add too much code & variables & listeners if we allow to change classes like you said.
Maybe we can think about some props to handle style (e.g border-radius, ...).