Added gray cmap to plot function
Added gray cmap to plot function

Closes #52

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