Add encoding support.

`slidify` accepts encoding so it is expected to work in MBCS locale.

I tested with success:
- Win 7 locale Japanese_Japan.CP932
    - input Rmd: CP932
    - input Rmd: utf8
- OS X Snow leapard
    - input Rmd: CP932
    - input Rmd: utf8

Both standalone and draft mode is working.

Anyway, please check it.

I'm very very very happy (omit, same as rstudio/markdown)