Add gray parameter to convert RGB to grayscale and plot grayscale images

[SarthakJariwala]

Implementation

# If data is a RGB image and gray is True, 
# convert to rgb to grayscale internally and plot image using a grayscle colormap
>>> isns.imgplot(data, gray=True)

# If data has only 2 dimensions and gray is True, plot image using grayscale colormap
>>> isns.imgplot(data, gray=True)

# If gray is True and cmap is not None, use cmap instead of grayscale colormap
# If data is RGB, conversion to grayscale will still be performed
>>> isns.imgplot(data, gray=True, cmap="deep")