i3 config: Try to use pavucontrol as the mixer if available

This falls back to alsamixer if pavucontrol isn't found in the $PATH

Note the sh -c here (and elsewhere) is because my $SHELL is fish which
won't work since it isn't Posix compliant <rant> and I wish applications
would stop assuming that an interactive $SHELL is Posix compliant
</rant>.