`echo` opt could support int and negative int

[jangorecki]

I often find it useful to exclude particular line(s) from what is being printed in the document.
For example set.seed() call is rather not very useful for an average reader of the document - whoever would like to reproduce the code, then they can find complete source of it. Therefore

    ```{r abc, echo=-1}
    set.seed(108)
    print("ok")
    ```

could exclude first row from printing to the output.

It possibly could be contributed by the community if you give a green light.