Automatically display default values of options in the help page (#1032)

Fixes #973

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

@@ -8,10 +8,11 @@ ARGUMENTS:
     [LEGS]    The number of legs
 
 OPTIONS:
-    -h, --help               Prints help information
-    -a, --alive              Indicates whether or not the animal is alive
+                             DEFAULT
+    -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
+        --agility <VALUE>    10         The agility between 0 and 100
 
 COMMANDS:
     lion <TEETH>    The lion command