Sanitise latex captions of kable() and figures

[ThierryO]

Using % and _ in captions with kable() or fig.cap were problematic when rendered to pdf. These are now sanitised with escape_latex()

[yihui]

The problem is that if I forcibly sanitize them, they may be problematic in other types of output if the source document format is R Markdown, because a character that is special to LaTeX is not necessarily special to another output format.

If your source document format is R Markdown, you may consider the feature of text references in bookdown for maximum portability: https://bookdown.org/yihui/bookdown/markdown-extensions-by-bookdown.html#text-references

[ThierryO]

The format of our source files is R Markdown/bookdown. We have been bitten by the examples below when rendering to pdf

```{r my-plot, fig.cap = "Estimate and 95% confidence interval}
my.plot()
```

```{r my-table}
kable(mtcars, caption = "Caption with some _ underscore")
```

The format is taken into account when sanitising the kable caption. That might not be the case for the fig.cap.

Could you explain why the Travis build fails? Because R CMD Check --as-cran passes on my machine.

[ThierryO]

Here is a minimal bookdown example that works with this PR. The Travis errors seem unrelated to me.

---
title: "Caption test"
author: "Thierry Onkelinx"
date: "24 juli 2017"
output: 
  bookdown::pdf_book: default
  bookdown::epub_book: default
  bookdown::html_document2: default
---

```{r load, include=FALSE}
library(knitr)
opts_chunk$set(echo = TRUE)
```

```{r pressure, echo=FALSE, fig.cap = "Pressure with percent % and underscore_"}
plot(pressure)
```

```{r cars}
kable(mtcars, caption = "mtcars with percent % and underscore_")
```

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