Fix bug where un-escaped curly braces can arrive in the styling_latex_font_size() function

[cgrandin]

The user could call kbl(df, align = "C{1.8cm}") and then later kable_styling() will break . This align value will arrive with the curly braces un-escaped in the styling_latex_font_size() function. I just added escaping for those curly braces, leaving the other curly braces that have been escaped alone by using look-behind regular expressions to look for curly braces without preceding backslashes.

Note that latex support for such a column definition looks like this:
\newcommand{\PreserveBackslash}[1]{\let\temp=\#1\let\=\temp}
\newcolumntype{C}[1]{>{\PreserveBackslash\centering}p{#1}}

These definable column definitions are highly useful!

[dmurdoch]

I'm just getting the error

Error in knitr::kable(x = x, format = format, digits = digits, row.names = row.names,  : 
  'align' must be a character vector of possible values 'l', 'r', and 'c'

Are you depending on other changes, or an unreleased version of knitr?