Simplified logic for both Linux and OS X

Avoiding the if statements, using exit code of `grep -q` instead.

Reverted the change that adds a space after the AC char. Opting for a
default value, allowing to override from one's profile.

plugins/available/battery.plugin.bash

@@ -4,20 +4,12 @@ about-plugin 'display info about your battery charge level'
 ac_adapter_connected(){
   if command_exists acpi;
   then
-    acpi -a | grep "on-line"
-    if [[ "$?" -eq 0 ]]; then
-      return 1
-    else
-      return 0
-    fi
+    acpi -a | grep -q "on-line"
+    return $?
   elif command_exists ioreg;
   then
-    local IOREG_OUTPUT=$(ioreg -n AppleSmartBattery -r | grep '"ExternalConnected"' | awk -F'=' '{print $2}')
-    if [[ "$IOREG_OUTPUT" == *"Yes"* ]]; then
-      return 0
-    else
-      return 1
-    fi
+    ioreg -n AppleSmartBattery -r | grep -q '"ExternalConnected" = Yes'
+    return $?
   fi
 }