use ffmpeg instead of mencoder

[collingreen]

Just use ffmpeg directly instead of going through mencoder

[Nzen]

I use /usr/local/sbin/ffmpeg2_8_1/ffmpeg -framerate 2 -pattern_type glob -i "*.png" -i "1611nn.mp3" "1611nn project.mp4" to post process the images. That will probably not produce an appropriate image sequence, if force capture has a distinct filename prefix or folder, as in issue [#8].