API update allowing library users to get the suggested file extension

associated with a particular file output driver.  The additional
plugin function is optional, so drivers that don't need to implement it
don't have to.  That's why we don't need to bump the plugin API version number.


git-svn-id: http://svn.xiph.org/trunk/ao@1946 0101bb08-14d6-0310-b084-bc0e0c8e3800