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.

tests/bison.in

@@ -26,6 +26,14 @@ BISON_PKGDATADIR=$abs_top_srcdir/data
 export BISON_PKGDATADIR
 
 stderr=tmp-bison.$$
+
+# If stderr is a tty, force --color=yes to simulate --color=auto
+# although we save and modify stderr.
+if test -t 2; then
+    set x --color=yes ${1+"$@"}
+    shift
+fi
+
 $PREBISON "$abs_top_builddir/src/bison" ${1+"$@"} 2>"$stderr"
 status=$?