add an initialize_store called before export

unused at this point, will be used by lua
lua : add a new callback to lua storage before any store happens

this is undocumented (but marked as such) because auto-generated
doc is not stable right now

the new function has the following prototype :
* storage : the storage object used
* format : the format object used
* images : a table of images to be exported
* quality : boolean, true if high quality export
* extra : the extra data table used by all export callbacks
returns : the list of images to export or nil.
  if nil (or no returned value) the original list will be used
  if an empty table is returned, no image will be exported,
  but finalize will still be called