talk-llama: optional wake-up command and audio confirmation

[Rakksor]

This adds two optional flags to talk-llama:

• -w TEXT / --wake-command TEXT: only process commands starting with the given text. This is basically a 1:1 copy of the always-prompt mode code from the command example.
• -ho TEXT / --heard-ok TEXT: A word or phrase said by the TTS to confirm that the text is being processed. This plays directly after whisper transcription to bridge the silence until llama generated an answer (useful for slower systems).

Example usage:

./talk-llama -mw ./models/ggml-tiny.en.bin \
             -ml ../llama.cpp/models/tinyllama-1.1b-chat-v1.0.Q4_K_M.gguf \
             -w "computer" \
             -ho "processing"

This will only answer to input starting with "computer" and say "processing" while waiting for an answer.