add Crop() sample code.

samples/crop.go

@@ -0,0 +1,29 @@
+package main
+
+import (
+	"os"
+	"path"
+	"runtime"
+
+	opencv "github.com/vanillahsu/go-opencv/opencv"
+)
+
+func main() {
+	_, currentfile, _, _ := runtime.Caller(0)
+	filename := path.Join(path.Dir(currentfile), "../images/lena.jpg")
+	if len(os.Args) == 2 {
+		filename = os.Args[1]
+	}
+
+	image := opencv.LoadImage(filename)
+	if image == nil {
+		panic("LoadImage fail")
+	}
+	defer image.Release()
+
+	crop := opencv.Crop(image, 0, 0, 50, 50)
+	opencv.SaveImage("/tmp/crop.jpg", crop, 0)
+	crop.Release()
+
+	os.Exit(0)
+}