xtable is suboptimal for live demos

[innermatrix]

We replaced kable with xtable because kable was creating problems with Brazil vignette output (see #12) but now we discovered that xtable is awful for inline rendering and live demos (because it prints HTML to R output chunk, which is useless). Investigate whether xtable has a mode that is useful or write a wrapper around xtable/kable that picks the better one, depending on the context.

[DanWeinberger]

I have replace the xtable refs with htmlTable, which seems more stable