title: Error when rendering tufte_html_book() with R/bookdown
body: <p>I'm trying to build a book with R/bookdown and I'm facing a problem when producing a html version using tufte_html_book or html_book: </p>
  
  <pre><code>Error in split_chapters(output, page_builder, number_sections, split_by,  : 
    The document must start with a first (#) or second level (##) heading
  </code></pre>
  
  <p>The book is correctly produced in gitbook or tufte_book2 output. 
  The document has four files <code>index.Rmd</code>, <code>01-Chap1.Rmd</code>, <code>02-Chap2.Rmd</code>, <code>03-Chap3.Rmd</code>. I like to keep things clear, so <code>index.Rmd</code> only contains the YAML headers, plus some R code but no heading, which is probably what yields the error. Is there a way to make the html outputs of bookdown behave similarly to gitbook or pdf output?</p>
  
  <p>Here is a small reproducible example: </p>
  
  <p>Index.Rmd:</p>
  
  <pre><code>---
  title: "Tufte Handout"
  subtitle: "An implementation in R Markdown"
  author: "JJ Allaire and Yihui Xie"
  date: "`r Sys.Date()`"
  site: bookdown::bookdown_site
  output:
    bookdown::tufte_html_book:
      toc: yes
  bookdown::tufte_book2:
      toc: yes
  ---
  
  ```{r setup1, include=FALSE}
  library(tufte)
  # invalidate cache when the tufte version changes
  knitr::opts_chunk$set(tidy = FALSE, cache.extra = 
  packageVersion('tufte'))
  options(htmltools.dir.version = FALSE)
  ```
  </code></pre>
  
  <p>01-Chap1.Rmd</p>
  
  <pre><code># Introduction
  
  The Tufte handout style is a style that Edward Tufte uses in his books and handouts. Tufte's style is known for its extensive use of sidenotes, tight integration of graphics with text, and well-set typography. This style has been implemented in LaTeX and HTML/CSS^[See Github repositories [tufte-latex](https://github.com/tufte-latex/tufte-latex) and [tufte-css](https://github.com/edwardtufte/tufte-css)], respectively. We have ported both implementations into the [**tufte** package](https://github.com/rstudio/tufte). If you want LaTeX/PDF output, you may use the `tufte_handout` format for handouts, and `tufte_book` for books. 
  For HTML output, use `tufte_html`. These formats can be either specified in the YAML metadata at the beginning of an R Markdown document (see an example below), or passed to the `rmarkdown::render()` function. See @R-rmarkdown more information about **rmarkdown**.
  </code></pre>