limit the state updates to a maximum updateRate

Van still provides the tools to limit the update rate. 

```JS
let {updateRate} = van
updateRate(100) 
```

This set´s the update rate to 100ms. state changes are delayed by up to 100 ms, before they are propagated to the DOM. If multiple changes are applied withi this time, only the last is updating the DOM.

Author: efpage

src/van.js

@@ -18,6 +18,7 @@ function end(cnt = 1) {
 }
 
 // This file consistently uses `let` keyword instead of `const` for reducing the bundle size.
+var updateRate = 0 // Initial update timeout 0 ms
 
 // Aliasing some builtin symbols to reduce the bundle size.
 let Obj = Object, _undefined, protoOf = Object.getPrototypeOf
@@ -35,7 +36,7 @@ let stateProto = {
     let s = this, curV = s._val
     if (v !== curV) {
       if (s.oldVal === curV)
-        changedStates = addAndScheduleOnFirst(changedStates, s, updateDoms)
+        changedStates = addAndScheduleOnFirst(changedStates, s, updateDoms, updateRate)
       else if (v === s.oldVal)
         changedStates.delete(s)
       s._val = v
@@ -110,5 +111,5 @@ let bind = (...deps) => {
   }
   return binding.dom
 }
-
-export default {add, tags, state, bind, begin, end}
+let updateRate = (t) => updateRate = t
+export default {add, tags, state, bind, begin, end, updateRate}