make it possible to specify runtime via --runtimes console argument

[adamsitnik]

The best way of testing the benchmarks for multiple runtimes is to pass target framework monikers via --runtimes console argument.

dotnet run -c Release -f netcoreapp2.0 --filter * --runtimes net471 netcoreapp2.0

Please keep in mind that:

• you should never try to run the benchmarks in Debug. To run benchmarks in Release, you need to pass -c Release to dotnet run command
• .NET Standard is an abstraction, you can't test the performance of .NET Standard. You can test the performance of runtimes which implement it
• Full .NET Framework always runs the code using the latest .NET Framework installed on a given machine. If you specify --runtimes net471 net472 it will run the code for the same Runtime anyway.
• you need to specify filter via --filter argument. It's a glob expression applied to full benchmark names: namespace.typeName.methodName. Run dotnet run -- --help to get help and some examples to learn more