General improvements.

• [Breaking] ProcessSupervisor.Start() is async.
• Add ExitKilled state for when a process was killed when it didn't shut down within time out (or if no timeout supplied). #96
• When non terminating process shuts itself down after a signal but exits with a non-zero exit code, the state is ExitWithError instead of ExitSuccessfully. #105
• Improved multi-threaded handling in ProcessSupervisor. #96
• Supports netstandard2.0, netcoreapp2.1, netcoreapp3.1, net5.0 #96

Thanks to @pgermishuys for contributions and feedback!