[Feature request] Extra flags for GUI applications embedding youtube-dl on Windows

[Faith001]

• I've verified and I assure that I'm running youtube-dl 2018.11.23

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Description of your issue, suggested solution and other information

Every time we get to conversion with ffmpeg, a console window opens up for the remainder of ffmpeg's operation due to the way Windows handles new processes being spawned.

Would it be possible for an extra extra option to be passed when embedding youtube-dl, denoting that the application is graphical so that the calls to the postprocessor can go from

https://github.com/rg3/youtube-dl/blob/d19600df07128c73ef7242af7e1cd8c819951aba/youtube_dl/postprocessor/ffmpeg.py#L164

https://github.com/rg3/youtube-dl/blob/d19600df07128c73ef7242af7e1cd8c819951aba/youtube_dl/postprocessor/ffmpeg.py#L199

to

handle = subprocess.Popen(cmd, stderr=compat_subprocess_get_DEVNULL(), stdout=subprocess.PIPE, stdin=subprocess.PIPE, creationflags=0x08000000)

and

p = subprocess.Popen(cmd, stdout=subprocess.PIPE, stderr=subprocess.PIPE, stdin=subprocess.PIPE, creationflags=0x08000000)

respectively.

Where 0x08000000 is CREATE_NO_WINDOW as per https://docs.microsoft.com/en-us/windows/desktop/procthread/process-creation-flags

This would also require a platform check since this would only work on Windows.
I've tried this and it works , would've made a pull request but I'm not too familiar with the codebase, just managed to track down where and how ffmpeg was being called to patch it for myself.