Add command line argument - video start and end frames

[azmaite]

Right now the df3d video making options include only -n (number of frames) - which means make a video from frame 0 to n.
It would be great to be able to specify the starting frame n0 and the end frame n1 instead, to make a video for a particular snippet and not just the start

[jasper-tms]

Perhaps we can keep -n 50 as "process the first 50 frames" but expand it so that -n 25 50 (two numbers instead of one number passed after -n) would mean "process frame 25 to frame 50". How does that sound?

[azmaite]

Sounds good to me, that way it is back-compatible with anyone's older code.