mentioning pander and ascii packages as alternative kable functions

As discussed at https://github.com/yihui/knitr/commit/057f41da16e503c2ffe991809b14021b10939a11#commitcomment-4556724, I added a few more characters to the `\seealso` sectinn of `kable` about my `pander` package:
  * special function: http://rapporter.github.io/pander/#tables
  * general S3 method: http://rapporter.github.io/pander/#generic-pander-method

PS: sorry for the lame updates in the `Rd` file, hopefully my Emacs did not scew up anything important. Please let me know if I should tweak something in this pull request.