Allow for an optional output in plugin multi

alsa-project/alsa-lib issue #379 was opened from wonbinbk:

Let's say I have 2 sound cards, card 0 and card 1. Card 1 is unplug-able from the system so it might not exist when applications read asound.conf.

I think it makes sense to add ability to specify an optional pcm device which if it doesn't exist won't break alsa.

``` ALSA lib ../../alsa-lib-1.2.6.1/src/confmisc.c:165:(snd_config_get_card) Cannot get card index for 1 ```

Issue URL : https://github.com/alsa-project/alsa-lib/issues/379 Repository URL: https://github.com/alsa-project/alsa-lib