[pfff] optionally include hack (.hhi) files when searching for php files

Summary: defaults to off

Test Plan:
elsewhere:

      lang=ml
      Tags_php.php_defs_of_files_or_dirs ~verbose:true ~include_hack:true
      (Lib_parsing_php.find_source_files_of_dir_or_files [dir] ~include_hack:true )

ran locally to validate tags generated:
./stags -lang hack /var/www/hack -o /tmp/derp

Reviewers: pad, jwatzman, julienv, eletuchy

Reviewed By: pad

CC: jan, joelp

Differential Revision: https://phabricator.fb.com/D1263293