If a device doesn't have a volume, don't crash

A virtual device can be a 7.1 device, which doesn't seem to have the
same sort of volume value
Remove duplication in default checking
If a sink has no mute value, don't display nothing

No mute value means 'no'
Add simple output that only shows volume and mute state of default sink

This mirrors the output of an awesomewm alsa bar
Merge pull request #1 from xentac/master

Some fixes that make my system work better