Move handling of SetLetterVowel() to language files
Contributes to:
Issue #218, https://github.com/espeak-ng/espeak-ng/issues/218
Changes:
language files have a new keyword letterVowel. It can be a latin alphabet from a-z or a hex value (used by bulgarian).
Errors in parsing the values are only reported, nothing is done about them.
About testing:
I haven't noticed any differences in the output audio with letterVowel set or unset in any tested language. The code seems to work and the correct bits seem to be set, but I don't know how to confirm from the final audio.
TODO:
1. Write better documentation in docs/voices.md
2. The code uses new_translator. Should it use translator instead?
Juho Hiltunen