re-created unprefixed version of reveal.css automatically using autoprefixer, to facilitate merging of changes to it

merge upgrade of highlight.js, add base styles to pre code, dont limit language detection in index.html #809

Correction of a small error

according to README, I don't include reveal.min.js.
Hence I believe it makes sense to not include reveal.min.css either.
Hence, examples added in index.html can give the feeling that they don't work yet.

.slides>section and .slides>section>section set the font-weight to
"normal". This overrides any font-weight setting a theme may set at
the body or .reveal level, and requires a theme author to also
specifically set the font-weight for .slides>section and
.slides>section>section. That's tedious and also counterintuitive.

Adopt a saner default by setting the font-weight to inherit.