add `options$engine.opts` here so users can pass more options to `coffee`, e.g. `-t`:

```r
  cmd = sprintf('%s -p -c %s %s', shQuote(options$engine.path %n% options$engine),
                options$engine.opts, shQuote(f))
```

BTW, is there a special reason for `-c`? I'm not familiar with `coffee`, but this option can be passed through `engine.opts` if necessary.