Add custom file name to capture_jpeg()

[aoyamaprod]

Thank you for developing the great library as always. This is an improvement request.
It would be better to let user assign the 'stem file name' for capture_jpeg() instead of fixed 'img0001.jpeg' file name. If we connect RTC to MEMENTO, I can add the datetime to the file name (e.g. img-YYYYMMDD-hhmmss-0001.jpeg).