Allow to hide Confirming Button on a CustomContent Dialog

## 🏗 Enhancement Proposal

Allowing MaterialDialog.Instance.ShowCustomContentAsync() to set confirmingText = Null , to hide the confirming Button.

The same way as dismissiveText do.

_You can remove the negative button by passing null to the dismissiveText parameter._

Optional : Create MaterialDialog.Instance.ShowCustomContentAlertDialogAsync() , same as ShowCustomContentAsync , without buttons ;)

### Pitch

Custom "ConfirmDialogs" are great. But Custom Alert Dialog are great too 🤙