Add documentation for ENV.argc and ENV.argv

docs/env.md

@@ -12,3 +12,9 @@ ENV.set("VAR_KEY", "VAR_VALUE")
 // list all env variables
 var list = ENV.keys()
 ```
+
+### Class Constants
+```swift
+var max_arg = ENV.argc - 1 
+var my_arg = ENV.argv[max_arg]
+```

src/cli/gravity.c

@@ -422,6 +422,7 @@ int main (int argc, const char* argv[]) {
     // create VM
     gravity_vm *vm = gravity_vm_new(&delegate);
 
+    // pass argc and argv to the ENV class
     gravity_env_register_args(vm, argc, argv);
 
     // check if input file is source code that needs to be compiled