GoProOverlay: added start/duration arguments to addOverlay.py.

Allows restriction of the add-overlay process to a part of the video, reducing processing time.

#aviation #GoProHero #python3 #gnumake

https://github.com/FrankAbelbeck/GoProOverlay/commit/6706d6a2672cefd3a26f3d2eace5a1ef3c1e989d