Suggested by Akim Demaille.  Fixes #69.

* doc/org/.dir-locals.el, doc/org/init.el.in,
wrap/python/tests/automata.ipynb: Set arrowhead and arrowsize.
* doc/org/autfilt.org, doc/org/dstar2tgba.org,
doc/org/ltl2tgba.org, doc/org/oaut.org: Adjust.

... or the pointer might be invalidated if the environments
changes.  Fixes #63.

* src/taalgos/dotty.cc, src/tgbaalgos/dotty.cc,
src/tgbaalgos/dtbasat.cc, src/tgbaalgos/dtgbasat.cc:
Copy the environment in strings instead.
* wrap/python/tests/automata.ipynb: Adjust comment.

* wrap/python/tests/automata.ipynb: Restrict dot size, and more
examples.

This implement several new options for --dot in order to
allow emptiness sets to be output as colored ⓿ or ❶...
Also add a SPOT_DOTDEFAULT environment variable.

* NEWS, src/bin/man/spot-x.x, src/bin/common_aoutput.cc,
src/bin/dstar2tgba.cc: Document the new options.
* doc/org/.dir-locals.el, doc/org/init.el.in: Setup
SPOT_DOTEXTRA and SPOT_DOTDEFAULT for all documents.
* doc/org/autfilt.org, doc/org/dstar2tgba.org, doc/org/ltl2tgba.org,
doc/org/ltldo.org, doc/org/oaut.org, doc/org/randaut.org,
doc/org/satmin.org: Adjust to this new setup.
* src/misc/escape.cc, src/misc/escape.hh (escape_html): New function.
* src/tgba/acc.cc, src/tgba/acc.hh (to_text, to_html): New method.
* src/tgbaalgos/dotty.cc: Implement the new options.
* src/tgbatest/readsave.test, wrap/python/tests/automata.ipynb: More
tests.
* wrap/python/spot.py: Make sure the default argument for
dotty_reachable is None, so that SPOT_DOTDEFAULT is honored.

* src/tgbaalgos/dotty.cc, src/taalgos/dotty.cc: Honnor the SPOT_DOTEXTRA
environement variable.
* src/tgbatest/readsave.test, wrap/python/tests/automata.ipynb: Test it.
* NEWS, src/bin/man/spot-x.x: Document it.

* wrap/python/spot.py: Introduce spot.translate (and
spot.formula.translate) as well, as a wrapper around the
spot.translator class.  Also implement spot.tgba.show()
to allow passing argument to dotty_reachable() before
the result is converted to SVG.
* wrap/python/tests/automata.ipynb: New test file.
* wrap/python/tests/Makefile.am: Add it.