Add mbstring detection

Running pickle without mbstring produces:

```
C:\pickle>php bin/pickle
PHP Fatal error:  Call to undefined function Hoa\Core\mb_internal_encoding() in
C:\pickle\vendor\hoa\core\Hoa\Core\Core.php on line 184

Fatal error: Call to undefined function Hoa\Core\mb_internal_encoding() in C:\pi
ckle\vendor\hoa\core\Hoa\Core\Core.php on line 184
```