Add node engine

Added node to the engine options. The markdown for node should use javascript as the language, so there is a small edit to `hooks-md.R` to check for that.

Example `.Rmd`:

    ```{r node-example0, engine="node"}
    var foo = 42
    console.log("The answer is ", foo);
    ```
    
    ```{r node-example1, engine="node", results="asis"}
    var foo = 42
    console.log("The answer is ", foo);
    ```
    
    ```{r node-example2, engine="node", results="hide"}
    var foo = 42
    console.log("The answer is ", foo);
    ```
    
    ```{r node-example3, engine="node", echo=FALSE}
    var foo = 42
    console.log("The answer is ", foo);
    ```

markdown:

    ```javascript
    var foo = 42
    console.log("The answer is ", foo);
    ```
    
    ```
    ## The answer is  42
    ```
    
    
    ```javascript
    var foo = 42
    console.log("The answer is ", foo);
    ```
    
    
    The answer is  42
    
    
    ```javascript
    var foo = 42
    console.log("The answer is ", foo);
    ```
    
    
    ```
    ## The answer is  42
    ```

oh, I did not notice you actually reverted 3c75f750632bf8e92ec756c9e624247e15cf0697 here... that is another reason that you should rebase against my master branch here.

I will redo 3c75f750632bf8e92ec756c9e624247e15cf0697, anyway.