added coffeescript engine

Added support for [CoffeeScript](http://coffeescript.org/). The `coffee` executable has an option to accept inline input `-e` and another option to print the compiled javascript `-p`. The usage would have to be:

```
    <script type="text/javascript">
    ```{r , engine="coffee", results="asis"}
        alert "Hi!"
    ```
    </script>
```

but I wonder if `results="asis"` could be set up as a default when using the `coffee` engine, and if the script tags could be auto-generated.