Tweak html_vignette

The new html vignette feature is pretty cool! I played with it and made a couple of changes:

- I do not think it makes sense to disable MathJax by default: when it is offline, the raw LaTeX code is displayed anyway; for those who dare write LaTeX code, it is still "readable"; I understand that Pandoc can convert some simple LaTeX expressions using HTML entities such as `\pi` to `&pi;`, but I do not think this little gain is worth it; on the other hand, I think P(offline) is a lot smaller than P(online), and I would rather manually add `mathjax: null` than having to bring it back `mathjax: default` in most cases;
- a few elements were centered: body, the title block, table, and figure with caption
- I tweaked the table style (I really dislike [too many border lines](http://en.wikipedia.org/wiki/Trypophobia) in tables), and added a table example so you can feel how it looks like

Feedback welcome! @hadley @jjallaire 
When is `#header` generated?