[racket] terminal-color package - output colored text to the terminal - comments before release

This is really cool.

I agree that a `printf`-color` would be handy.

I notice that adding it would require adding it 3 times, to each of
the 3 plugins in private/.

I wonder if each plugin could instead supply just its variation of
`display-color`?  Because IIUC `displayln-color`, `print-color`, and
`write-color` (and `printf-color` and so on) could all be implemented
on top of that, up in main.rkt?

For example, private/off.rkt could provide just this:

 (define (display-color v #:fg fg #:bg bg)
   (display v))

Now main.rkt can dynamic-require just that, from the appropriate
private/ plugin, and implement everything else in terms of that:

 (define (displayln-color v #:fg fg #:bg bg)
   (display-color v #:fg fg #:bg bg)
   (newline))

 (define (print-color v #:fg fg #:bg bg)
   (define out (open-output-string))
   (print v out)
   (display-color (get-output-string out) #:fg fg #:bg bg))

 (define (write-color v #:fg fg #:bg bg)
   (define out (open-output-string))
   (write v out)
   (display-color (get-output-string out) #:fg fg #:bg bg))


That makes each private/ plugin simpler. Hmm, the above stuff
in main.rkt is still kind of repetitious. We could make that
simpler:

 (define (my-x f)
   (λ (v #:fg fg #:bg bg)
     (define out (open-output-string))
     (f v out)
     (display-color (get-output-string out) #:fg fg #:bg bg)))

 (define displayln-color (my-x displayln))
 (define print-color (my-x print))
 (define write-color (my-x write))