Add ExitStatus to Command.run()

The main difference for now is the removal of the --nohup
(slightly hacky) option in TestCommand. Now a TestCommand
can signal to its caller that it wants to be hung up.

Fixes gh-975

spring-boot-cli/src/test/java/cli/command/CustomCommand.java

@@ -28,8 +28,9 @@ public class CustomCommand extends AbstractCommand {
 	}
 
 	@Override
-	public void run(String... args) throws Exception {
+	public ExitStatus run(String... args) throws Exception {
 		System.err.println("Custom Command Hello");
+		return ExitStatus.OK;
 	}
 
 }