adds options class.output and class.source for html documents

[ijlyttle]

Addresses #1333

This work is just beginning - I am opening the PR now so that @yihui and @AmeliaMN can pull me back towards the right path should I go the wrong way.

• code
  • adapt render_markdown() in hooks-md.R
    • class.source
    • class.output
    • clean out extraneous comments in hooks.t.output
• documentation
  • make PR to knitr-examples
  • docs (docs/options/index.md)
• add a name to DESCRIPTION/Contributors

If you prefer to keep all the feedback until the whole PR is "done", I'll send a signal when I think I am there.

FWIW, here's a gist with the Rmd file I've been using to test things.

[yihui]

• Don't worry about the duplicated code. I'll take care of it since it is simple enough.
• The documentation can be added here (as you have found out): https://github.com/yihui/knitr/blob/master/docs/options/index.md No need to add vignettes or Rd.
• No need to add tests under tests/. An example in the knitr-examples repo will be great, though, and that can serve as a test.
• Then add your name to DESCRIPTION and done.

[ijlyttle]

Will do - should not be much longer - thanks!

[ijlyttle]

Done, methinks (mehopes). Thanks for the navigation through the process, @yihui!