Allow changing opacity of the background mask as well as hiding it

[ferusinfo]

In the project that I am working on, I was required to hide - and in some cases change its opacity - the background mask that is displaying below the overlay.

This pull request is adding that funcionality to the main library. Hope you can merge it and therefore we will be able to use the official repository instead of the fork.

Thanks! :)

[mrackwitz]

I'd propose to collapse the functionality into one single property dimmedBackgroundColor.

/**
The color used to dim the entire area of the superview when the overlay is shown.

The default value is translucent black with an opacity of 40%.

Set this property to `nil` to prevent a background color change when the overlay is shown.
*/
+@property (nonatomic, copy) UIColor *dimmedBackgroundColor UI_APPEARANCE_SELECTOR;

What do you think?

[ferusinfo]

That is a good call - I haven't thought about this honestly :+1: