feat: add vignette filter to helpers.c and integrate with CLI and docs

Author: sam131725

helpers.h

@@ -14,5 +14,5 @@ void reflect(int height, int width, RGBTRIPLE image[height][width]);
 
 // Blur image
 void blur(int height, int width, RGBTRIPLE image[height][width]);
-//
+// Vignette image
 void vignette(int height, int width, RGBTRIPLE image[height][width]);