Support of dotnet core

### Description

When trying to add Suave.Swagger to the dotnet core app the following error is appeared:
```
warning NU1701: Package 'Suave.Swagger 2.3.0' was restored using '.NETFramework,Version=v4.6.1' instead of the project target framework '.NETCoreApp,Version=v2.1'. This package may not be fully compatible with your project.
```