Have 'action function' accpet an argument if it there

If an action function has one argument, the action function's first argument will be the selected candidate name.

For example:

```cl
(ac-define-source my-test-1
  '((candidates . '("test1" "test2" "test3"))
    (action . (lambda ($selected) (message (format "----- %s -----" $selected))))))
(add-to-list 'ac-sources 'ac-source-my-test-1)
```

When "test1" completed with RET key, a message "----- test1 -----" is going to display on the mini buffer.