Add different colour for public holidays

[dgvirtual]

Would you be interested in adding a method to mark days as holidays (use another default colour)? I could submit such pull request.

It would involve adding one more class to css, adding property and method:

    protected $holidays = [];
    /** <...> */
    
    public function setHoliday($date)
    {
        $parsed = $this->parseDate($date);
        if (! $parsed) {
            throw new InvalidArgumentException('invalid date');
        }

        $this->holidays[$parsed->format('Y-m-d')] = true;
    }
    ```
    
And then sligthly modifying the render() method.