Add property for dark mode

[Séraphin Hochart]

Add-on to #41

Great work on this library! Still the most reliable out there

I am still using an older version, (1.4.1) where just tinting the view with a dark color was still making the view darker.

Right now, If I want to use the latest version(s), I need to go everywhere in my code and add black overlays everywhere which follow my frame changes etc, which feels like a hack.
iOS 8 blur does it out of the box (with a dark mode), but this library is still useful if you want to support for iOS 7.

Thanks!

[Guglielmo Faglioni]

+1

[James An]

+1

[Chris Edwards]

+1, this would be extremely useful!