[suggestion] Add option for 2-pass in windows shell

Currently the 2-pass command only works in linux systems. I'd like to suggest an option to select Windows shell for the 2-pass script. It would work like so:
Replace the `/dev/null` with the Windows equivalent `NUL` virtual path ([see this SO answer](https://stackoverflow.com/a/27773642/4526052)). ffmpeg asks for permission to overwrite this file, so an always overwrite flag on the first pass would be wonderful.

Also in PowerShell, the bash `&&` operator does not work. The equivalent `-and` operator messes with the pipes, so the best alternative I could find is this:
```powershell
ffmpeg -i input.mp4 -pass 1 -f mp4 -y nul; if ($?) { ffmpeg -i input.mp4 -pass 2 utput.mp4 }
```
NOTE: Not sure how this would look in command-prompt (cmd.exe). This would only work on powershell