Improve generate_unaccent_rules.py to handle composed characters whose base
is another composed character rather than a plain letter.  The net effect
of this is to add a bunch of multi-accented Vietnamese characters to
unaccent.rules.

Original complaint from Kha Nguyen, diagnosis of the script's shortcoming
by Thomas Munro.

Dang Minh Huong and Michael Paquier

Discussion: https://postgr.es/m/CALo3sF6EC8cy1F2JUz=GRf5h4LMUJTaG3qpdoiLrNbWEXL-tRg@mail.gmail.com