modifiers/snap: add limits to snap grid function

interact.createSnapGrid({
  x: 50,
  y: 50,
  limits: {
    left: 100,
    right: 400,
    top: Infinity,
    bottom: Infinity,
  }
});

Close #208

src/modifiers/snap.js

@@ -217,6 +217,12 @@ const snap = {
 
 interact.createSnapGrid = function (grid) {
   return function (x, y) {
+    const limits = grid.limits || {
+      left  : -Infinity,
+      right :  Infinity,
+      top   : -Infinity,
+      bottom:  Infinity,
+    };
     let offsetX = 0;
     let offsetY = 0;
 
@@ -228,8 +234,8 @@ interact.createSnapGrid = function (grid) {
     const gridx = Math.round((x - offsetX) / grid.x);
     const gridy = Math.round((y - offsetY) / grid.y);
 
-    const newX = gridx * grid.x + offsetX;
-    const newY = gridy * grid.y + offsetY;
+    const newX = Math.max(limits.left, Math.min(limits.right , gridx * grid.x + offsetX));
+    const newY = Math.max(limits.top , Math.min(limits.bottom, gridy * grid.y + offsetY));
 
     return {
       x: newX,