Automatic dark mode

I am using MRProgress in several apps. In all apps it seems to be a good idea to choose the same background color like the navigation bar (if one apparent) as the default background color of the progress view. So I changed all default coloring implementation to automatically change to a dark coloring (by simply inverting the colors) when the status bar style is set to `LightContent`.

Maybe you want to consider to integrate this into MRProgress or think about a similar solution. I've read https://github.com/mrackwitz/MRProgress/issues/59 and couldn't find a solution for a background tint yet. So I came up with an automatic solution. Hope you like it.