Provide the user with a way to customize the ffmpeg command used to render their video

[WampyCakes]

Description of proposed feature

#1144 introduces a change to the ffmpeg command used to create a GIF. While it seems to handle every scene I've tested well, I am sure there will be scenes that benefit from different settings (such as a different dithering). I think that we should either provide a way to set in a script config["ffmpeg_command"] or at least providing through the config more ways to control GIF (read: ffmpeg) flags.

How can the new feature be used?

Examples of these are adjusting the current palletegen and palleteuse in #1144 or allowing the user to add flags like the following to reduce file size:

scale=320:-1:flags=lanczos

Additional comments

I am not sure of the exact form this should take regarding the dynamic properties used in the ffmpeg command. Perhaps just passing an array with placeholders and then executing exactly what the user provides would be enough. (Like interpreting file_list as the literal file_list variable)

[leotrs]

I am not opposed to this, but it sounds like it will require some non-trivial refactoring of scene_file_writer.

Dynamic properties can already be achieved for paths saved in config (see ManimConfig.get_dir).

[WampyCakes]

This customizability would also fix #786.