Fix set margin left before offsetWidth prop is assigned

There are times where `this.el.offsetWidth`'s value is `0` and the position gets centered wrong at [index.js#L238](https://github.com/component/dialog/blob/a56a5178dca1c95b6128ca76d63a1c3ff0ce145c/index.js#L238).

I set a timeout right after DOM append so `this.el.offsetWidth` retrieves the actual *offsetWidth* value.