[Gitbook configurations]]

```r
gitbook_config = function(config = list()) {
  default = list(
    sharing = list(
      github = FALSE, facebook = TRUE, twitter = TRUE, google = FALSE,
      linkedin = FALSE, weibo = FALSE, instapaper = FALSE, vk = FALSE,
      all = c('facebook', 'google', 'twitter', 'linkedin', 'weibo', 'instapaper')
    ),
    fontsettings = list(theme = 'white', family = 'sans', size = 2),
    edit = list(link = NULL, text = NULL),
    history = list(link = NULL, text = NULL),
    down