add support for --html

* bootstrap.conf: We need the "execute" module.
* src/files.h, src/files.c (spec_html_file, html_flag): New.
* src/getargs.h, src/getargs.c (--html): New.
* src/print-xml.h, src/print-xml.c (print_html): New.
* src/main.c: Use them.
* tests/output.at, tests/report.at: Check --html.

src/reader.c

@@ -735,6 +735,7 @@ prepare_percent_define_front_end_variables (void)
       muscle_percent_define_default ("lr.default-reduction", "accepting");
     free (lr_type);
   }
+  muscle_percent_define_default ("tool.xsltproc", "xsltproc");
 
   /* Check %define front-end variables.  */
   {