Make kable() escape LaTeX special characters

When a table contains special characters such as `%` or `_`, they are not escaped by `kable()`, making the LaTeX file compilation fail.

For example, the following `Rnw` file will not compile : 

    \documentclass{article}
    \begin{document}
    
    <<>>=
    library(knitr)
    kable(data.frame(x=c("10%","5%"),col_name=c("3_8","40_6")), format="latex")
    @
    
    \end{document}

This very small patch just apply `escape_latex` to column names and table elements.