Change coffeescript flags so that code is evaluated, not just compiled

Currently when using `engine="coffee"` the code is run with the `-p` flag which prints the compiled code instead of the output.

For example, the following `.Rmd` file

    ```{r engine="coffee"}
    x = 42
    console.log "The answer is ", x
    ```

compiles to the following markdown

    ```coffee
    x = 42
    console.log "The answer is ", x
    ```
    
    ```
    ## (function() {
    ##   var x;
    ##
    ##   x = 42;
    ##
    ##   console.log("The answer is ", x);
    ##
    ## }).call(this);
    ```

By removing the `-p` flag the code is evaluated and the resulting markdown is

    ```coffee
    x = 42
    console.log "The answer is ", x
    ```
    
    ```
    ## The answer is  42
    ```

which seems to be the desired output given my experience using `knitr` with R and python.