Add default file path and export options in preferences

Users would be able to export multiple assets from a file much faster if they could set a default file path as well as default export options in the add-on's preferences. If that were done, we could add an option to not even open the File Browser when exporting from the Outliner.

From: https://blendermarket.com/inbox/conversations/87099

Start by tracing how the add-on defines preferences and how exports from the Outliner open Blender's File Browser. Identify where export options and the destination path are selected. Done should mean preferences can provide both defaults and an export can use them without opening the File Browser.

Written by the indexing model from the issue text.