Improve todo#PrependDate to follow format

Add logic to place the completion date after the priority, if it exists,
when completing a task.

autoload/todo.vim

@@ -62,7 +62,11 @@ function! todo#RemovePriority()
 endfunction
 
 function! todo#PrependDate()
+    if (getline(".") =~ '\v^\(')
+	execute "normal! 0f)a\<space>\<esc>l\"=strftime(\"%Y-%m-%d\")\<esc>P"
+    else
         normal! 0"=strftime("%Y-%m-%d ")
+P
     endif
 endfunction