Pass options to ffmpeg securely

[itsrachelfish]

Currently ffmpeg is being called by node as a string, command and arguments combined. This lead to a severe bug which locked up the server and clients when files were added to the playlist with "&" in the filename.

Options passed to commands run by node should be sanitized, like the way they are in fishy's rainbow module - https://github.com/itsrachelfish/fishy-bot/blob/master/modules/rainbow.js#L73-L84

[mozai]

https://nodejs.org/api/child_process.html#child_process_child_process_spawn_command_args_options with shell=false