[mod_elasticsearch] Add support for usage sequence sorting

[loetie]

Currently it is not possible to do a search with a sort argument "seq". The sort argument is ignored and done on relevance. Within ginger foundation this is for example used in the page.collection.tpl for getting the collection items.

It would be better if we could support the default zotonic search argument.

[ddeboer]

@loetie Can you give an example of a search query where sorting by seq is useful?