Resolve "Bug: positionAllowed allows fields outside the grid"

Closes #10 (closed)

src/DragGrid.vue

@@ -144,7 +144,7 @@ export default {
   },
   methods: {
     positionAllowed(x, y, key) {
-      if (x < 0 || y < 0) return false;
+      if (x <= 0 || y <= 0) return false;
       if (x > this.cols) return false;
       if (y > this.rows) return false;