Files are now quoted with ' instead of " now as to avoid more edge cases
that come from using quotation marks. Now only ' have to be escaped in
the item paths.
Using the ogg container allows to also embed cover images while still
using the opus codec.
Also fixed an edge case that would make the script fail if the first
thing it iterated on was a regular file which didn't require conversion
if the parent directory wasn't already created.
