Now we have 4 targets that uses HTML code:

HTML
HTML5
XHTML
XHTML Strict

Their current file extensions for the output file are:

html
html5
xhtml
xhtmls

It's good to have it separated, but it's not practical since in the real world 
.html is the preferred in all cases.

We need a way for the user to specify the default file extension she wants to 
use for a target.

We already have --outfile, but it's not funny to enter the full filename 
everytime when you just want to change the extension and use the current source 
basename (%%infile).

I suggest a new %!extension setting, that can be use in a file by file basis, 
or to the used in the global .txt2tagsrc file.

%!extension(html5): html
%!extension(xhtml): html
%!extension(xhtmls): html

%!extension(bbcode): bb
%!extension(creole): wiki

And so on. A sister command line option --extension could also be used.