Implement new functionality: Replacement URI creator function

This introduces a new way in which to get the URIs to the replacement PNG
images, by using a JavaScript function.  This function may analyse the jQuery
element which contains the SVG image to be replaced and then return a URI to the
replacement PNG image.

* This short-cuts out the remote API server call but requires that the hosting
  platform is able to serve the replacement PNGs with no pre-notification that
  they are needed, just a request to their URI.

* The function is null by default (meaning that the default functionality will
  remain, preserving backwards-compatibility) - it is presented as an option
  which may be enabled if desired.

* This functionality may be useful for offline testing use-cases or in
  specialised scenarios in which the replacement images are alrady created and
  hosted.