espeak binary on MacOS

[lawrenceakka]

tts.py hardcodes the name of the espeak binary as espeak-ng. But ...

• On Windows, the binary is espeak-ng.exe, so the existing code works
• On Linux it is espeak, though espeak-ng is sometimes installed as a symlink, so the existing code may work.
• On MacOS, if built directly from source it will be espeak-ng, but if installed with homebrew (ie the majority of cases)
  it will be espeak, so the existing code will not work.

I'm happy to write a PR for this, but wonder if it is preferable to have the name of the binary as a configurable option in glados_config.yml, or whether the initialisation should attempt to detect which is installed. Which would you prefer?