Lazy loading script-tags

[9uenther]

The changes let me now load script-tags dynamically, when a defined parent element is shown in the viewport. You can also omit the parent id, in that case the real parent is used to load the script.

Following example shows the usage with google-maps:

<div id="map"></div>
<script class="b-lazy" data-parent="map" data-src="https://maps.googleapis.com/maps/api/js?key=mySuperSecretAPIkey&#038;callback=initMap" async defer></script>