Feature two color area series

Hi! This is my first pull request. 

I've added a new feature - a new type of series - TwoColorAreaSeries with 3 examples in the example library.

The Issue #10150 - exists only in codeplex ( https://oxyplot.codeplex.com/workitem/10150 ). 

I had to draw two-color area chart with baseline with area filled with one color above the line, and area filled with another color below that line. That kind of chart was important for me, so I'd created for myself new class derived from TwoColorLineSeries. I think that's a good feature and it would be nice if that can be part of oxyplot. 

As a next step, I'm going to add WPF wrapper for that kind of series, so that it can be used in xaml code.

Can you check this, please?