Add support for one pass bitrate for encoders

[Chris Griffith]

From discord @chriss

Hello, can you set Bitrate mode on x264 and x265 to 2 options ? Encode 1 pass and 2 pass because I still want to use one pass if i use Bitrate Mode

[Chris Griffith]

Added in https://github.com/cdgriffith/FastFlix/releases/tag/5.6.0