added utility script to generate image thumbnail for shiny aps etc
added utility script to generate image thumbnail for shiny aps etc

AntonSax/plantcv,danforthcenter/plantcv,stiphyMT/plantcv,danforthcenter/plantcv,stiphyMT/plantcv,jshoyer/plantcv,AntonSax/plantcv,danforthcenter/plantcv,jshoyer/plantcv,stiphyMT/plantcv