Modified Command.argument to let the user provide a method call for runtime learning of options value. This will let the compiler check at compile time

src/cligen/command/argument.cr

@@ -35,7 +35,11 @@ module CliGen
       {% end %}
       {% if options %}
         {% options = options.resolve if options.is_a? Path %}
-        {% raise "ERROR : CliGen::Command.argument(#{name}) : Provided options must be an ArrayLiteral" unless options.is_a? ArrayLiteral %}
+        {% if options.is_a? Call %}
+        {% elsif options.is_a? ArrayLiteral %}
+        {% else %}
+        {% raise "ERROR : CliGen::Command.argument(#{name}) : Provided options must be an ArrayLiteral or a runtime method call to retrieve data" %}
+        {% end %}
       {% end %}
       {% if format %}
         {% format = format.resolve if format.is_a? Path %}