Fixes#497.
* spot/twaalgos/dot.cc: Implement this option.
* tests/core/ltl2tgba.test, tests/core/randaut.test: @ is
now a valid option for --dot, use something else.
* tests/python/aliases.ipynb: New file.
* tests/Makefile.am, doc/org/tut.org: Add it.
* doc/org/hoa.org: Mention aliases.
* NEWS: Mention this new feature.
