Recognize multipage DjVu MIME type

There is new image/vnd.djvu+multipage MIME type for DjVu documents
in shared-mime-info.
This MIME type is used if Evince has to guess type of the document.
Evince states that it doesn't support it in such case.
This is reproducible if the opened document doesn't have an extension.

https://bugzilla.gnome.org/show_bug.cgi?id=754467