add support for running under a particular `RuntimeIdentifier`

[baronfel]

I have a program that uses Asp.Net Core, but on Mono, and it requires a runtime-specific build (ie dotnet build -r osx-x64) in order to use Kestrel due to framework- and os-level dependencies, so I can't use it with fcswatch right now. Would you accept the addition of the -r|--runtime flag to specify the runtime to build and/or run with?

[humhei]

Would you accept the addition of the -r|--runtime flag to specify the runtime to build and/or run with?

Yes.