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.

bootstrap.conf

@@ -22,7 +22,7 @@ gnulib_modules='
   calloc-posix close closeout config-h c-strcase
   configmake
   dirname
-  error extensions
+  error execute extensions
   fdl fopen-safer fstrcmp
   getopt-gnu
   gettext-h git-version-gen gitlog-to-changelog