Update vunit.js
Update vunit.js

Proposing an optional callback (`onChange`) that gets called when vunit detects viewport changes.
the callback gets a parameter, the object of viewport dims. ex: `{width: 1174, height: 993}`.

if you're doing expensive calls here, may want to use >https://remysharp.com/2010/07/21/throttling-function-calls>
joaocunha/vunit,JosePedroDias/vunit,JosePedroDias/vunit,joaocunha/vunit