Expose new transcription options

Some generation parameters that were available in the CTranslate2 API but not exposed in faster-whisper:

• repetition_penalty to penalize the score of previously generated tokens (set > 1 to penalize)
• no_repeat_ngram_size to prevent repetitions of ngrams with this size

Some values that were previously hardcoded in the transcription method:

• prompt_reset_on_temperature to configure after which temperature fallback step the prompt with the previous text should be reset (default value is 0.5)

Other changes

• Fix a possible memory leak when decoding audio with PyAV by forcing the garbage collector to run
• Add property duration_after_vad in the returned TranscriptionInfo object
• Add "large" alias for the "large-v2" model
• Log a warning when the model is English-only but the language parameter is set to something else