[Issue #39] Ensure interim videos are generated with right format.

This tweak should ensure that interim videos created in order to have a title page are created using the same format as the output video.  It has been tested using avconv 0.8.5 but needs to be tested also with FFmpeg.

Cautionary note: when I try and create a video with title page, the notes aren't included in the final video.  I'm not sure if this is a fault of my patch, of avconv, or of ly2video.