diagnostics: use libtextstyle for colored output

Bruno Haible released libtextstyle, a library for colored output based
on CSS.  Let's use it to generate colored diagnostics, provided
libtextstyle is available.

See
https://lists.gnu.org/archive/html/bug-gnulib/2019-01/msg00176.html
https://lists.gnu.org/archive/html/bison-patches/2019-02/msg00073.html
https://lists.gnu.org/archive/html/bison-patches/2019-02/msg00084.html
https://lists.gnu.org/archive/html/bison-patches/2019-03/msg00007.html

* bootstrap.conf (gnulib_modules): Use libtextstyle when possible.
* data/diagnostics.css: New.
* src/complain.c (begin_use_class, end_use_class, flush)
(severity_style, complain_init_color): New.
Use them.
* src/getargs.c (getargs_colors): New.
(getargs): Use it.
Skip --color and --style.
* src/location.h, src/location.c (location_print): Use a style.

* tests/bison.in: Force --color=yes when stderr is a tty.
* tests/local.at: Disable colors during the test suite.
* tests/input.at: Adjust expectations to the extra options passed on
the command line.

bootstrap.conf

@@ -28,6 +28,7 @@ gnulib_modules='
   gpl-3.0 hash inttypes isnan javacomp-script
   javaexec-script
   ldexpl
+  libtextstyle
   malloc-gnu
   mbswidth
   non-recursive-gnulib-prefix-hack