Allow user to set ffmpeg path via environment variable

[lesservehicle]

Hi! I just ran into an issue on macOS, running this with a Homebrew installed version of ffmpeg. The function as-is assumes that the shell is aware of the location of ffmpeg via $PATH, but depending on how you execute the node, your path may be wiped out. I've seen other nodes use this sort of method as a workaround to that.

[hesa2020]

Just a note "/usr/bin/ffmpeg" wont work on windows.
I would say keep "ffmpeg" and let the user fix his own pathing by modifying environment variable correctly?

If it gets remove from $PATH you are not setting it up correctly.

As a note if you export PATH in a console it will be temporary in that console window until closed but if you edit the path file /etc/paths it should be permanent.

[jeffy5]

Thanks for your contribution. Yes, I think @hesa2020 is right. Maybe you should keep the default path as "ffmpeg" so that it works better on all platforms.

@lesservehicle Please change "/usr/bin/ffmpeg" to "ffmpeg". Then I will merge it, thanks.