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/Expectations/Help/Default_Without_Args.Output.verified.txt

@@ -0,0 +1,16 @@
+DESCRIPTION:
+The lion command.
+
+USAGE:
+    myapp <TEETH> [LEGS] [OPTIONS]
+
+ARGUMENTS:
+    <TEETH>    The number of teeth the lion has
+    [LEGS]     The number of legs
+
+OPTIONS:
+    -h, --help               Prints help information
+    -a, --alive              Indicates whether or not the animal is alive
+    -n, --name <VALUE>
+        --agility <VALUE>    The agility between 0 and 100
+    -c <CHILDREN>            The number of children the lion has

test/Spectre.Console.Cli.Tests/Expectations/Help/Default_Without_Args_Additional.Output.verified.txt

@@ -0,0 +1,19 @@
+DESCRIPTION:
+The lion command.
+
+USAGE:
+    myapp <TEETH> [LEGS] [OPTIONS]
+
+ARGUMENTS:
+    <TEETH>    The number of teeth the lion has
+    [LEGS]     The number of legs
+
+OPTIONS:
+    -h, --help               Prints help information
+    -a, --alive              Indicates whether or not the animal is alive
+    -n, --name <VALUE>
+        --agility <VALUE>    The agility between 0 and 100
+    -c <CHILDREN>            The number of children the lion has
+
+COMMANDS:
+    giraffe <LENGTH>    The giraffe command

test/Spectre.Console.Cli.Tests/Expectations/Help/Greeter_Default.Output.verified.txt

@@ -0,0 +1 @@
+Hello World