Serialize a list of subtitles according to the SRT format, with optional time padding.

def srt_formatter(subtitles, padding_before=0, padding_after=0):
    """
    Serialize a list of subtitles according to the SRT format, with optional time padding.
    """
    sub_rip_file = pysrt.SubRipFile()
    for i, ((start, end), text) in enumerate(subtitles, start=1):
        item = pysrt.SubRipItem()
        item.index = i
        item.text = six.text_type(text)
        item.start.seconds = max(0, start - padding_before)
        item.end.seconds = end + padding_after
        sub_rip_file.append(item)
    return '\n'.join(six.text_type(item) for item in sub_rip_file)