#98: Move .remark-slide-content out of .remark-slide.

Before, you had to specify .remark-slide .remark-slide-content { ... } to
override remark default CSS. As both classes are prefixed with 'remark-'
there's really no excuse for not flattening the hiearchy. Now, all you
need to do is to specify .remark-slide-content.

Classes like .left, .right and so on are still nested inside
.remark-slide-content, however. Maybe they should be extracted sometime too.