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/getargs.h

@@ -36,6 +36,7 @@ extern char const *include;
 
 extern bool header_flag;                /* for -d/-H */
 extern bool graph_flag;                 /* for -g */
+extern bool html_flag;                  /* for --html */
 extern bool xml_flag;                   /* for -x */
 extern bool no_lines_flag;              /* for -l */
 extern bool token_table_flag;           /* for -k */