Fixes #4318: `yii\helpers\Html::ul()` and `ol()` will return an empty list tag if an empty item array is given

1. Added proper handling and support of the symlinked directories.
2. Added $options parameter.

issue #746

- Added protected `\yii\Inflector::transliterate()` that could be replaced with custom translit implementation.
- Added proper tests for both intl-based slug and PHP fallback.
- Removed character maps for non-latin languages.
- Improved overall slug results.
- Added note about the fact that intl is required for non-latin languages to requirements checker.

Improved `yii\helpers\Inflector::slug` to support more cases for Russian, Hebrew and special characters

No changelog line since feature was introduced after beta

Fixes #2906: Added support for using conditional comments for js and css files registered through asset bundles and Html helper