add option to output current volume to stdout

[hovnatan]

I needed an option to output the current volume to stdout for i3blocks

[hastinbe]

i3blocks has a script for outputting volume information to the statusline. i3-volume sends a signal for it to update when the volume changes (SIGRTMIN+10 by default). It also displays MUTED when it is muted

[hastinbe]

FYI: this was added in v2.2.0 under commit 8f420b4. Example for i3blocks:

[i3volume]
label=
command=~/i3-volume/volume -o i3blocks
interval=once
signal=10
color=#b8bb26
separator_block_width=10
markup=pango