Better Bootstrap 4 support and Former::plaintext() security fix

Added

• Add CSS class to the label of a group, with the addLabelClass() method (#604)

Changed

• Better Bootstrap 4 support
• Breaking change: Escape HTML value of plaintext field by default (#605)
  You can disable this new behavior with the escape_plaintext_value former config option set to false.
  In your former config file config/former.php, you can enable or disable this feature:

<?php

return [
    //...

    // Whether Former should escape HTML tags of 'plaintext' fields
    // Enabled by default
    //
    // Instead of disabled this option,
    // you should use the 'HtmlString' class:
    //  Former::plaintext('text')
    //      ->forceValue(
    //          new Illuminate\Support\HtmlString('<b>HTML data</b>')
    //      )
    'escape_plaintext_value' => true,

    //...
];