fnExtractFrameToJPG enable file names to digits. #5

[ryoja]

Hi.
I tried to fix #5.
If you specify a file name in such a code, "my_frame_001.jpg, my_frame_002.jpg, ..." files are created.

         video.fnExtractFrameToJPG ('/path/to/save_your_frames', {
             frame_rate: 1,
             number: 5,
             file_name: 'my_frame_%03d'
         }