Implement FLAC codec in video encode

[kubinka0505]

As this one reports ValueError, this would allow lossless audio encoding

from moviepy import editor as mp

clip = mp.ColorClip((100,) * 2, color = (0,) * 3, duration = 2)
audio_clip = mp.AudioFileClip("in.wav")
clip = clip.set_audio(audio_clip)

clip.write_videofile(
    "out.mp4",
    fps = 24,
    codec = "libx264",
    audio_fps = clip.audio.fps,
    audio_codec = "flac",
    rewrite_audio = False, # ignored
    ffmpeg_params = ["-strict", "-2"],
)