dict.pronounce: pass phonemes as an out parameter

The tts::dictionary::pronounce function returned the list of
phonemes from the pronunciation of the given entry. This limited
the usage of the API because:

  1.  It was not possible to tell if a pronunciation lookup failed
      or if there was an entry with no phonemes.

  2.  The pronunciation function could not handle more complicated
      lookup used by hyphenated words, multi-word entries or
      compound words.

This change does not support (2) within the pronounce method, but
makes it easier to support.