%!target and %!includeconf are the only commands that do not support the 
optional target specification, such as:

%!csv(html): foo.csv

Where foo.csv will only be included when the target is HTML.

OK
   %!target: html
   %!includeconf: foo.conf

NOT OK
   %!target(txt): html
   %!includeconf(html): foo.conf

Today, the last two commands will be executed as if the user has not specified 
the target. So it will always be true.

This can confuse the user and bring unexpected problems.

Txt2tags should raise an error when user tries to specify a target in commands 
that do not support it.