now work with MBCS (tested on CP932) on windows.

Under locale=Japanese_Japan.CP932 (windows default in Japan), `markdownToHTML` sometimes fails.
This is because the line, `sub(perl=TRUE)` marks `html` as `UTF-8` (see `Encoding(html)`), and next call of `sub(fixed=TRUE)` a few lines below fails due to invalid multibyte string.

I think this is quite a simply bug, because `fixed=TRUE` in other calls of `sub` with a literal pattern.
Even worse we cannot run `knitr::knit2html` due to this bug.
This is very very serious... 

I hope this will be merged and `markdown` will be updated on CRAN soon.
Thanks,

kohske