Parallelize validate-transcript.sh to boost execution speed x20

With the increasing amount of transcript and SVGs, validate-transcript was taking over a big minute on my hardware. It was too long to wait for it when I had to merge all the MR for incoming lang for episode 34, longer than renderfarm.

This modification parallelize the loop that enters in a episode folder. Execution time is now.... 4 seconds on my machine :-)

The drawback is the requirement of the parallal dependency; and not be 100% Bash script only anymore. But that's a small price, imo, for getting this acceleration.

I'm not sure if Framagit C.I. use this script. It's probably where the parallelisation might become "problematic". That's why I'll put this change in a MR for review before merging.

If it is problematic (ping @gunchleoc : you might be able to predict this better than anyone here), I'll restore the script for webcomics/0_transcript, and move this parralelised version to peppercarrot/tools repository. Shouldn't be a problem, but better to attempt this here before splitting into two scripts that does the same thing.