bak plugin: add help on 0 arguments

plugins/bak/bak.load

@@ -4,9 +4,19 @@ function __bak_name
   echo "$arg.(date +"%Y%m%d_%H%M%S").bak"
 end
 
+function __bak_help
+  echo -e \
+"Usage:
+  $argv[1]bak SOURCE..."
+end
+
 function __bak
   set program $argv[1]
+  if [ (count $argv) -gt 1 ]
     for arg in $argv[2..-1]
       eval $program $arg (__bak_name $arg)
     end
+  else
+    __bak_help $argv[1]
+  end
 end