Add syntax highlighting for Sweave documents

R/S blocks in Sweave documents are parsed using r.lang.
Inline R/S blocks (within \Sexpr{}) are shown as comments.

https://bugzilla.gnome.org/show_bug.cgi?id=620241
sweave.lang: some fixes

The R-block context is not included into another context containing an
<end> tag, so "extend-parent" is useless.

"style-inside" is also useless, because no style is applied to the whole
context, the styles are applied to the different components (start, end,
the R code).