Enable C# 8 Nullable Reference Types

[RehanSaeed]

Describe the feature

Add C# 8 Nullable Reference Types.

[VictorioBerra]

Please make this an optional flag to enable.

[RehanSaeed]

Well, this is in the framework project. So no flags here.

In the templates, I think having an optional feature will be very difficult. It will mean a huge number of #if FOO directives. Not sure. Need to play with it to see.

[VictorioBerra]

The big thing is that it changes the entire way someone programs. Its almost like writing a whole new language. Only a small fraction of people are using it every day.

[RehanSaeed]

Fixed by #276