Ignore comments in REPL

Before:

```console
$ ./stylus --repl
> // Lorem ipsum
error: Cannot call method 'toString' of undefined
> /* Lorem ipsum */
=> [object Object]
> /* Lorem */ ipsum
=> ipsum
> Lorem /* ipsum */
=> [object Object]
```

After:

```console
$ ./stylus --repl
> // Lorem ipsum
> /* Lorem ipsum */
> /* Lorem */ ipsum
=> ipsum
> Lorem /* ipsum */
=> Lorem
```