Add Unity command line arguments

[BlueFisher]

Adding addition Unity command line arguments.
I had to build multiple executables for each scene again if I updated ml-agents. It was really tedious. So I would like to write a InitialScene script to load scenes depending on the command line arguments.

public class InitialScene : MonoBehaviour {
    void Start() {
        List<string> commandLineArgs = new List<string>(Environment.GetCommandLineArgs());
        int index = commandLineArgs.IndexOf("--scene");
        if(index == -1) {
            File.WriteAllText("error.log", $"No --scene in {Environment.CommandLine}");
            Application.Quit();
        }
        else {
            string sceneName = commandLineArgs[index + 1];
            SceneManager.LoadScene(sceneName);
        }
    }
}

Then I only have to build once.

[CLAassistant]

All committers have signed the CLA.

[shihzy]

CC: @harperj