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.

data/local.mk

@@ -15,7 +15,8 @@
 ## along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 dist_pkgdata_DATA =                             \
-  data/README.md
+  data/README.md                                \
+  data/diagnostics.css
 
 skeletonsdir = $(pkgdatadir)/skeletons
 dist_skeletons_DATA =                           \