AsciiDoc additions

Back in August last year I offered to have a go at writing some hooks so knitr could generate AsciiDoc.  I'm very good at procrastinating, but I've finally gotten around to writing them.

It would be great if you could take a look at the patch to make sure I'm heading in the right direction.  (My knitr knowledge is still rather patchy, so I'm not sure exactly which bits need to be written.)

Thanks,
Richie.