Add subtitle format and font color in word_options dict

[MatteoFasulo]

Refactored iterate_result function to include a new subtitle_format option that modifies the style of subtitles. The function now iterates through the subtitles and formats the subtitle text based on the subtitle_format option. If the option is not provided, the default value of None is used. The value None will produce at most a warning when reading it from third part software (like FFMPEG) since there will be an empty HTML tag inside <None>.

Also a font_color option allow to insert an hex color in format #FFFFFF (white) to change the font color while producing the .srt file. If not specified, it uses the default white color.

The iterate_result function in utils.py now includes a new subtitle_format option that modifies the style of subtitles. The available options are:

• "u": Subtitles are underlined.
• "b": Subtitles are bolded.
• "i": Subtitles are italicized.

To use this option, simply include "subtitle_format": "u", "subtitle_format": "b", or "subtitle_format": "i" in the word_options dictionary.

Example of usage:

import whisper
from whisper.utils import get_writer 

audio = './audio.mp3'
model = whisper.load_model(model='small')
result = model.transcribe(audio=audio, language='en', word_timestamps=True, task="transcribe")

# Set VTT Line and words width
word_options = {
    "highlight_words": False,
    "max_line_count": 1,
    "max_line_width": 42,
    "subtitle_format": "b", # bold word highlighting
    "font_color": "#fff000", # yellow font color
}
vtt_writer = get_writer(output_format='vtt', output_dir='./')
vtt_writer(result, audio, word_options)