Optional path to the ffmpeg binary when initialising the class

Can you make the binary path optional when creating an instance and add it to the `execute` method?

For example:

```py
stream = (
    FFmpeg()
    .option("y")
    .input(audio_file)
    .input(image_file)
    .output("Output.mp3", {...})
    .execute(executable="ffmpeg/path/bin.exe")
)
```