[5.5] Blade : add else in the custom if directive

[bmichotte]

This PR adds theelse in the custom if directive to blade.

Example use-case :

<?php
Blade::if('instanceof', function ($object, $class) {
    return $object instanceof $class;
});
?>

(this example can not work as it, but you have the idea)

Actually, we have to use the @instanceof this way :

@instanceof($object, App\User::class)
   You're a user
@endinstanceof

@instanceof($object, App\Product::class)
   You're a product
@else
  I don't know what you are !
@endinstanceof

This PR allows us to write the following

@instanceof($object, App\User::class)
   You're a user
@elseinstanceof($object, App\Product::class)
   You're a product
@else
   I don't know what you are !
@endinstanceof

Tests are provided.

[ludo237]

This can be done by the user by extending blade itself

[bmichotte]

@ludo237 The if also can be done by the user :)

Also, this is more convenient because we don't have to write this kind of directives

Blade::directive('instanceof', function($expression) {
      $exploded = explode(',', $expression);
      $object = trim(array_shift($exploded));
      $class = trim(array_shift($exploded));
      return "<?php if ($object instanceof $class): ?>";
});
Blade::directive('elseinstanceof', function($expression) {
      $exploded = explode(',', $expression);
      $object = trim(array_shift($exploded));
      $class = trim(array_shift($exploded));
      return "<?php elseif ($object instanceof $class): ?>";
});
Blade::directive('endinstanceof', function() {
      return "<?php endif; ?>";
});