% symbol stripped from ffmpeg command

[SamarRizvi]

% symbol gets stripped out from resulting ffmpeg command (tested in Windows) causing error when a person tries PHPVideoToolkit for multiple output like frame every x seconds, etc.

[SamarRizvi]

@buggedcom If time permits, can you please merge this pull request. I have to manually change code whenever I do composer update and stuff due to this.

[buggedcom]

Merged, but not had time to check thoroughly so will have to trust you that it works.

[buggedcom]

released under 2.2.4