Change Piper TTS demo to accept text instead of phonemes

[robertknight]

Add a naive text-to-phoneme converter based on a dictionary built with the piper-phonemize PyPI package. This converter is naive in the sense that it just ignores words that don't appear in the dictionary, and doesn't deal with numbers, punctuation etc. Nevertheless, it can be used to translate simple user-specified sentences.

The provided en-us.txt dictionary was created using the pronounciation-dict.py script with the /usr/share/dict/words file on my system.

See also https://github.com/robertknight/rten/issues/194