Add an argument to specify the path to the ImageMagick executable

On other operating systems, or if a user is unable to add ImageMagick to path, an argument to specify the path to the ImageMagick executable would be useful, so that the user is not required to install ImageMagick to the gallery repository or edit the script to use ImageMagick. (e.g. `python auto_gallery.py -magickpath "C:\Users\username\AppData\Local\Programs\ImageMagick\magick.exe"`) If the path is not specified, then the script will simply run the command `magick`.