Reorder FFMPEG_VideoWriter command arguments

[ThePhonon]

stream options must be provided before the '-i' option to affect that stream.
The existing code caused '-an' (no audio) to be applied to the next stream. This causes no audio to be generated if the next stream is the audio stream for the clip.

e.g. On Windows:

import os
os.environ["IMAGEIO_FFMPEG_EXE"] = "path\to\newest\ffmpeg.exe"
from moviepy.editor import VideoFileClip
video = VideoFileClip("my_file_with_audio.mp4").subclip(0, 10)
video.write_videofile("created_clip.mp4")

will generate a video file with no audio.

• If this is a bugfix, I have provided code that clearly demonstrates the problem and that works when used with this PR
• I have added a test to the test suite, if necessary
• I have properly documented new or changed features in the documention, or the docstrings
• I have properly documented unusual changes to the code in the comments around it
• I have made note of any breaking/backwards incompatible changes

[tburrows13]

Closes #876 #934 and possible #1032.