Include the [named] model explicitly in LaTeX commands

[bpj]

The \fcolorbox command of xcolor has a nasty "feature":

If only the first optional argument is given, it specifies the color model for both colors.

which means that the LaTeX markup as currently produced by this filter causes a "missing number" error! The "named" model has to be included explicitly:

% These are equivalent:
\fcolorbox[HTML]{framecolor}{boxcolor}{text}
\fcolorbox[HTML]{framecolor}[HTML]{boxcolor}{text}

% FIXME so this (as currently generated BREAKS:
\fcolorbox[HTML]{FF0000}{yellow}{text}

% and has to be replaced by this, which WORKS
\fcolorbox[HTML]{FF0000}[named]{yellow}{text}

Although this is a bit of overkill I will change the filter to always include [named] before a named color both to keep the filter code simple and to be safe.