Add four additional arguments to function

The arguments are:
 - create_movie (boolean, FALSE)
 - frame_rate (numeric, 6)
 - IM_path (character)
 - ffmpeg_path (character)
Begin conditional structure for generating movie file
Generate a list of PDF files that were created
Obtain a file name for each PDF that was to be generated
Generate a string vector of PDF file names
Add comment to script
Begin loop for processing PDF files in the 'PDF_list' object
Convert PDF files to JPEG files using ImageMagick
Construct the movie output name
Generate the movie file using ffmpeg