Limit `FfmpegCommand.simple()` to a single input

[matthew-carroll]

Currently FfmpegCommand.simple() takes a List of inputs. However, the FFMPEG docs state: "Simple filtergraphs are those that have exactly one input and output, both of the same type."

https://ffmpeg.org/ffmpeg.html#Simple-filtergraphs

Unless I'm missing something, we should probably change inputs to a singular input.