to_rtf returns a string representing an RTF table. The fc_tables attribute of the
returned string will contain the fc_tables object that was passed in (or autocreated).
print_rtf prints the string and returns NULL.
Limitations
rmarkdown's rtf_document can't yet print out customized color tables, so custom fonts
and colors won't work in this context.
rotation() can only be 90 or 270, i.e. text going up or down.
Details
RTF files use a single per-document table for colors, and one for fonts. If you are printing
multiple huxtables in a document, you need to make sure that the font and color table is
set up correctly and that the RTF tables refer back to them. See rtf_fc_tables().