Ignore disabled monitors for -m

Don't check monitors without dimensions in xrandr's output.
Addresses #26.

tdrop

@@ -244,6 +244,7 @@ update_geometry_settings_for_monitor() {
 		monitors_info=$(xrandr --query | awk '/ connected/ {gsub("primary ",""); print}')
 		while read -r monitor; do
 			monitor_geo=$(echo "$monitor" | awk '{print $3}')
+			if [[ $monitor_geo =~ ^[0-9]+x[0-9]+\+[0-9]+\+[0-9]+$ ]]; then
 				x_begin=$(echo "$monitor_geo" | awk -F '+' '{print $2}')
 				y_begin=$(echo "$monitor_geo" | awk -F '+' '{print $3}')
 				x_width=$(echo "$monitor_geo" | awk -F 'x' '{print $1}')
@@ -255,6 +256,7 @@ update_geometry_settings_for_monitor() {
 					current_monitor=$(echo "$monitor" | awk '{print $1}')
 					break
 				fi
+			fi
 		done <<< "$monitors_info"
 	fi