Support prompt for QuickPick as well as InputBox

[tetchel]

I would like the createQuickPick and showQuickPick APIs to support a prompt field, the same way InputBox does, so I can show more information to the user that does not disappear when they begin to type.

[TylerLeonhardt]

@tetchel valid feature request... in the meantime, you can possibly use the title field for now.