backDropClass to ngbModalOptions

[jasedwards]

I suppose this is a feature request more than a bug. I was reading issue #818 and backdrop and window class was mentioned but it appears only the ability to add a window class was implemented. I have scenarios where I sometimes need a dark backdrop and other times a lite backdrop. I don't see any way to do one or the other. a backdropClass on ngbModalOptions would be nice unless there is already some way to do this