Adding initial page feature

Hi there,

I work at Lumis [http://www.lumis.com.br] and here we'll start to use a fork of your code to our publications viewer app, whose nickname is Poligno. We found useful to set an initial scroll to an element, so I'm sending you this pull request to thank you for your code and collaborate too.

Best regards,
=P
This doesn't check whether the siezes property has been provided

> `vs-size-property="propertyName"` - a property name of the items in collection that is a number denoting the element size (in pixels)