Z-SAMPA is a more complex specification than Kirshenbaum, X-SAMPA,
and CXS. Additionally, it specifies phonemes such as palatal and
velar trills that are not defined by IPA and in the context of the
velar trill is marked as being impossible.

As such, Z-SAMPA will not be supported in eSpeak NG for now. That
may change in the future once support for the other transcription
schemes has been implemented.

This makes it clearer that both variant and private-use tags are
used.

According to page 5 of [RFC5645](https://tools.ietf.org/html/bcp47#page-1-5),
variant subtags must have 5 to 8 characters. A 9-character subtag is
sytactically ill-formed and should be rejected by conforming parsers.

Technically, BCP47 does not allow the use of unregistered variant subtags.
For `fonkirsh`, I’ve filed a
[language variant subtag registration request](https://mailarchive.ietf.org/arch/msg/ietf-languages/1IrLl3n4wJ4Fr1xXV34QquJn8Bc)
with the IETF. If IETF rejects my request, we could probably add
Kirshenbaum to Unicode’s registry for [RFC6497](https://tools.ietf.org/html/rfc6497).

Not sure what to do about the other non-registered variants. If they’re
commonly used, it might be an option to register them as well, but
they seem to be even more exotic than Kirshenbaum, and we wouldn’t to
bloat the BCP47 registry too much. If espeak-ng really wants to handle these
alphabets, it would probably make sense to use BCP47 private-use extensions,
such as `x-fonzsamp`. According to IETF BCP47, private-use subtags can have
up to (but not more than) 8 characters, so I’m shortening these too.

This reverts commit 39994150ba.

This reverts commit 9689cc8860.

The stray space confuses GitHub’s markdown parser.

The "Place of Articulation" documentation was moved to the
phonemes.md documentation, but the table of content entry was
not removed.

This reverts commit 0f55204522.

This breaks using voice files like mb-de5-en. It could also use
named values for the stress rules (e.g. 'first-syllable').

Keyword stressRule in voices.c handles setting langopts->stress_rule.

tr_languages.c still contains the setting for some languages that don't have a language file yet.

Contributes to:
Issue #218, https://github.com/espeak-ng/espeak-ng/issues/218

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?