feat: add option to use custom espeak data path

[thewh1teagle]

Add option to use custom espeak data path
To use with https://github.com/thewh1teagle/espeakng-loader

from phonemizer.backend.espeak.wrapper import EspeakWrapper
from phonemizer import phonemize
import espeakng_loader

EspeakWrapper.set_library(espeakng_loader.get_library_path())
EspeakWrapper.set_data_path(espeakng_loader.get_data_path()) # or './espeak-ng-data' 

phonemes = phonemize('Hello')
print('Phonemes: ', phonemes)

By the way, I have to say that using attributes and static variables with the same names, like _ESPEAK_DATA_PATH, self._data_path, and data_path, is extremely confusing. I really don’t understand why this approach is used.