Add full UTF8 support to the Str::slug method

I have made a request to this problem before in [Pull request #6204](https://github.com/laravel/framework/pull/6204) .
The current ```Str::slug()``` method returns an empty string for strings like ```سلام دنیا```, this is due to converting the string to ```ASCII``` and also using ```mb_strtolower``` function.