Suggested by Matthias Heizmann. Fixes#109.
* NEWS: notify the new option
* THANKS: add Matthias Heizmann
* bin/autfilt.cc: add new option --accept-word=WORD which filters
automata that accept WORD
* doc/org/autfilt.org: add an example of the new option
* tests/Makefile.am: add core/acc_word.test to the list of test files
* tests/core/acc_word.test: test some uses of the new option
