2.0.0

1. Now targets .NET Standard 2.0 only
2. CRS model objects are gone (only pre-IETF versions of GeoJSON used them exactly that way)
3. Extension methods for "id" are gone (downstream libraries can add their own versions that don't require partying on a global variable in this library to make it work for them)
4. FeatureCollection now inherits from Collection<IFeature> instead of composing one.
5. Added a GetOptionalValue method to IAttributesTable to help simplify several use cases, such as GPX.