Add adelay filter

I'd like to add the [adelay filter](https://ffmpeg.org/ffmpeg-filters.html#adelay) option allow the audio sync to be changed.

Here's an example command

> ffmpeg -i input.mp4 -af "adelay=delays=150:all=1" ouput.mp4

I think using the `all` option is simplest, otherwise a delay needs to be set for every audio channel (two for stereo, like `adelay=150|150`).