Add "format" to ArrayGenerator options

[mlocati]

Examples:

1. without format (or "normal" format):

   <?php return array (
     'domain' => NULL,
     'plural-forms' => 'nplurals=2; plural=n != 1;',
     'messages' => 
     array (
       '' => 
       array (
         'Foo' => 'Bar',
         'PluralKey' => 
         array (
           0 => 'One',
           1 => 'Two',
         ),
       ),
     ),
   );

2. with "pretty" format (the old pretty option)

   <?php
   
   return [
       'domain' => null,
       'plural-forms' => 'nplurals=2; plural=n != 1;',
       'messages' => [
           '' => [
               'Foo' => 'Bar',
               'PluralKey' => [
                   'One',
                   'Two',
               ],
           ],
       ],
   ];

3. with the new "compact" format

   <?php return ['domain'=>null,'plural-forms'=>'nplurals=2; plural=n != 1;','messages'=>[''=>['Foo'=>'Bar','PluralKey'=>['One','Two']]]];