Show option hooks example to strip HTML in alt tex

[cderv]

yihui/knitr#2290 (comment)

knitr::opts_hooks$set(fig.cap = function(options) {
  if (is.null(options$fig.alt)) options$fig.alt = options$fig.cap
  options$fig.alt = xfun::strip_html(options$fig.alt)
  options
})