title: text subscript in kable col.names and header
body: <p>I cannot seem to produce an html table using the kableExtra package with column names or headers that contain subscripts.  I have tried multiple approaches but it just keeps showing the output without subscript.
  Using the mtcars data set as an example:</p>
  
  <pre><code>data(mtcars)
  library(kableExtra)
  mtcars %&gt;% filter(mpg==21.0) %&gt;% select(c(1:2)) %&gt;%  kable("html",col.names=c("m~pg~","c\\textsubscript{yl}"),escape=F) %&gt;% kable_styling()
  </code></pre>
  
  <p>I am trying to subscript the last two letters of the selected column names in this example for this html format table.  I have also tried using "$m~pg~$" and "$m_{pg}$" and "m\\textsubscript{yl}"</p>