add parallaxAdjacentItemOpacity to ILayoutConfig to set opacity on parallax backgrounded items

[o0JoeCool0o]

Is your feature request related to a problem? Please describe.
My requirements to implement a UI Mockup is to have the side parallax items semi transparent.
Describe the solution you'd like
adding a simple configMode attribute called parallaxAdjacentItemOpacity would be swell!

Describe alternatives you've considered
hijacking the render and setting the opacity manually / creating a fork and doing it myself just for my project.

Additional context
Just like in the header image on npmjs