HOWTO: add top border radius

[Frezyx]

👋 Hi, that's cool package! It makes very easy date input!
😪 But in design of the app that I'm currently creating exist only rounded ModalBottomSheet widgets.
👀 Is there any way to add border radius for DatePicker ?
Like this:

[BelfDev]

@Frezyx I just created a PR with the feature you requested :)

#282