features writer -- support pulmonic consonants

This change reworks the feature phonemeset writer to better format
the features. It currently only supports the pulmonic consonants
table phonemes without any diacritics or suprasegmentals.
Don't expose createExplicitFeaturePhoneme*.

These functions do not need to be directly exposed as they can be
accessed via:

    cainteoir::tts::createPhonemeReader("features")
    cainteoir::tts::createPhonemeWriter("features")