Added implementation and doc string

doc/examples/plot_seam_carving.py

@@ -0,0 +1,19 @@
+from skimage import io, data
+from skimage import transform
+from skimage import color, filters
+from matplotlib import pyplot as plt
+
+def custom_sobel(img):
+    if img.ndim == 3:
+        img = color.rgb2gray(img)
+
+    return filters.sobel(img)
+
+img = data.coins()
+out  = transform.seam_carve(img, 'vertical', 80, energy_func = custom_sobel)
+out  = transform.seam_carve(out, 'horizontal', 70, energy_func = custom_sobel)
+
+io.imshow(out)
+plt.figure()
+io.imshow(img)
+io.show()