Allow configuring the clr-loader runtime to use via environment variables
Configuring the runtime has essentially three common cases:
- .NET Framework
- Mono with a globally accessible
libmono.so - .NET Core
To make this easier to use (and hide clr_loader a bit), I propose that we introduce a new environment variable PYTHONNET_RUNTIME that configures what happens if one just imports clr, and a corresponding pythonnet.configure function with some simple parameters.