existing styles (character vector) where first element
represents the style to use for title 1, second element represents
the style to use for title 2, etc.
Details
Function addTitle need to know which styles
are corresponding to which title level (1 ; 1.1 ; 1.1.1 ; etc.).
When template is read, function docx try to guess what
are theses styles. If he do not succeed, an error occured saying
'You must defined header styles via map_title first.'. In that
case, run styles(...) to see what are available styles, then
map_title to indicate which available styles are meant to be
used as header styles.