terminal title with current directory

I'm currently doing this:

```
(define old-prompt (current-prompt-function))
(define (my-prompt #:last-return-value [last-ret #f]
                   #:last-return-index [last-ret-n 0])
	(printf "\033]0;~a\007" (path->string (current-directory)))
	(old-prompt)
)

(current-prompt-function my-prompt)
```