Show help for default command (#953)

Shows help if the default command was called without any arguments but has required arguments.
This way it does not hinder execution of a default command without any required arguments.

When called without args:
- Default command with required params -> Show help
- Default command with required params and additional commands -> Show help including additional commands
- Default command without required params -> Execute command

test/Spectre.Console.Cli.Tests/Data/Commands/GreeterCommand.cs

@@ -0,0 +1,17 @@
+using Spectre.Console;
+
+public class GreeterCommand : Command<OptionalArgumentWithDefaultValueSettings>
+{
+    private readonly IAnsiConsole _console;
+
+    public GreeterCommand(IAnsiConsole console)
+    {
+        _console = console;
+    }
+
+    public override int Execute([NotNull] CommandContext context, [NotNull] OptionalArgumentWithDefaultValueSettings settings)
+    {
+        _console.WriteLine(settings.Greeting);
+        return 0;
+    }
+}