Clarify 'composer update' message + generate *.php for Grav >= 1.7

[pamtbaau]

I would like to suggest two changes:

1. From own experience and from post on forum, the message displayed after creating a 'new-plugin' is confusing when user is not familiar with composer. It is not clear that user must change directory into the folder of the new plugin. See also issue #61

   When user creates plugin 'myplugin', new message would be:

   'Please run `cd user/plugins/myplugin` and `composer update` to initialize the autoloader'

2. Since Grav 1.7+ is now the default version people download, I would like to suggest the generated code is for Grav >= 1.7 and show comment when required version of Grav is < 1.7

   public static function getSubscribedEvents(): array
   {
       return [
           'onPluginsInitialized' => [
               // Uncomment following line when plugin requires Grav < 1.7
               // ['autoload', 100000],
               ['onPluginsInitialized', 0]
           ]
       ];
   }
   

[w00fz]

Thanks, looks good