cli: allow user cli options to override defaults for fallback audio encoder
Solves: https://forum.handbrake.fr/viewtopic.php?f=10&t=28074&sid=68d8c5bfffd7081625bcd56fb7c4dad1


git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@6236 b64f7644-9d1e-0410-96f1-a4d463321fa5