Improve word-level timestamps heuristics

Some recent improvements from openai-whisper are ported to faster-whisper:

• Squash long words at window and sentence boundaries (https://github.com/openai/whisper/commit/255887f219e6b632bc1a6aac1caf28eecfca1bac)
• Improve timestamp heuristics (https://github.com/openai/whisper/commit/f572f2161ba831bae131364c3bffdead7af6d210)

Support download of user converted models from the Hugging Face Hub

The WhisperModel constructor now accepts any repository ID as argument, for example:

model = WhisperModel("username/whisper-large-v2-ct2")

The utility function download_model has been updated similarly.

Other changes

• Accept an iterable of token IDs for the argument initial_prompt (useful to include timestamp tokens in the prompt)
• Avoid computing higher temperatures when no_speech_threshold is met (same as https://github.com/openai/whisper/commit/e334ff141d5444fbf6904edaaf408e5b0b416fe8)
• Fix truncated output when using a prefix without disabling timestamps
• Update the minimum required CTranslate2 version to 3.17.0 to include the latest fixes