high cpu usage

On my machine, there is high cpu usage on pages that use holder. Traced it back to these lines in index:

```
// Done to prevent 100% CPU usage via aggressive calling of requestAnimationFrame
setTimeout(function () {
global.requestAnimationFrame(visibilityCheck);
}, 10);
```

Since this is a recursive call, can we only make these calls on DOM changes? Or pehaps a way for the developer to turn off the timeout, and manually call Holder when a change has occurred.

Start in index at the setTimeout/requestAnimationFrame recursion around visibilityCheck and reproduce the high CPU usage on a page using Holder. Compare the reported continuous checks with the requested DOM-change or manual-update behavior; done means the excessive CPU use is resolved without breaking placeholder updates. No test file is named in the issue.

Written by the indexing model from the issue text.