Mixvideoconcat

- Add option to limit frame rate during autodetection
- Improve ffmpeg version support
- Add prefer verical video option

- Skip resize for uniform videos.
- Add frame rate detection (convert to highest).
- Add possibility to override FFMPEG constants by means of environment variables.
- Switch to default CRF 23 (can be overrided by `FFMPEG_CRF` environment variable).

- Add possibility to disable ffmpeg output.
- Add possibility to turn off deinterlace and/or stabilization.
- Fix MOV files processing.

- Initial release.