The issue at ruby-av/paperclip-av-transcoder#3
is caused, at least on my system with Paperclip 4.2, by the format
parameter of the form: ".FMT".  This caused the case statement to not
match any of the desired formats!  It's likely that other paperclip
version don't pass the dot before the format extension, but it seems
likely that the behavior may differ depending on the version.

This change looks for the end of the format string to match using a
regex, and should now properly find the format, allowing thumbnails
to be generated (as well as the proper switches for non image params)

It was failing because it was manually adding the output_type,
while the Base#add_destination does it as well, causing the
options, aac and experimental to be added twice.

While this fixes the issue, I wonder if it might be better
to make the option setting idempontent?  Although with command-
line switches, I suppose one never knows when repeated arguments
are valid and necessary.