osx: add more arguments to `itunes vol` command (#7845)

- Get the iTunes volume without an argument
- Shift up / down iTunes volume

plugins/osx/osx.plugin.zsh

@@ -236,7 +236,19 @@ function itunes() {
 			opt="$opt track"
 			;;
 		vol)
-			opt="set sound volume to $1" #$1 Due to the shift
+			local new_volume volume=$(osascript -e 'tell application "iTunes" to get sound volume')
+			if [[ $# -eq 0 ]]; then
+				echo "Current volume is ${volume}."
+				return 0
+			fi
+			case $1 in
+				up) new_volume=$((volume + 10 < 100 ? volume + 10 : 100)) ;;
+				down) new_volume=$((volume - 10 > 0 ? volume - 10 : 0)) ;;
+				<0-100>) new_volume=$1 ;;
+				*) echo "'$1' is not valid. Expected <0-100>, up or down."
+				   return 1 ;;
+			esac
+			opt="set sound volume to ${new_volume}"
 			;;
 		playlist)
 		# Inspired by: https://gist.github.com/nakajijapan/ac8b45371064ae98ea7f
@@ -299,7 +311,7 @@ EOF
 			echo "\tmute|unmute\tcontrol volume set"
 			echo "\tnext|previous\tplay next or previous track"
 			echo "\tshuf|shuffle [on|off|toggle]\tSet shuffled playback. Default: toggle. Note: toggle doesn't support the MiniPlayer."
-			echo "\tvol\tSet the volume, takes an argument from 0 to 100"
+			echo "\tvol [0-100|up|down]\tGet or set the volume. 0 to 100 sets the volume. 'up' / 'down' increases / decreases by 10 points. No argument displays current volume."
 			echo "\tplaying|status\tShow what song is currently playing in iTunes."
 			echo "\tplaylist [playlist name]\t Play specific playlist"
 			echo "\thelp\tshow this message and exit"