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?

docs/voices.md

@@ -30,7 +30,7 @@
   - [stressAmp](#stressamp)
   - [intonation](#intonation)
   - [dictmin](#dictmin)
-
+  - [letterVowel](#letterVowel)
 ----------
 
 A Voice file specifies a language (and possibly a language variant or
@@ -425,3 +425,9 @@ Used for some languages to detect if additional language data is
 installed. If the size of the compiled dictionary data for the language
 (the file `espeak-ng-data/*_dict`) is less than this size then a
 warning is given.
+
+### letterVowel
+
+	letterVowel <letter>
+
+Used for some languages to handle a certain letter as a vowel instead of consonant.