[DX] Re-work generate:bundle for simple app bundles #290

@weaverryan please review my pull request ;)
With my comment here: https://github.com/sensiolabs/SensioGeneratorBundle/pull/299/files#r16630753, it should actually default to `$input->getOption('shared')` - just like you see here: https://github.com/sensiolabs/SensioGeneratorBundle/blob/master/Command/GenerateDoctrineEntityCommand.php#L162