Support rendering offset

Support starting the rendering a few seconds into the recording, then looping around from the end to the beginning.

The idea behind this is that it generates a more reasonable thumbnail when sharing the gif on various platforms. The viewer won't typically notice this if the output is running as a continuous loop.

Proposed syntax:

# Take the first 10% of the video and place it after the end.
Set LoopOffset 10%
# Take the first 10 frames and place them at the end.
# The first frame of the video will be the 11th frame.
Set LoopOffset 10

Sounds great!

@maaslalani We can use the -start_number option to set the starting frame in ffmpeg image2 and move the frames which are before it to end of the sequence by renaming their file names.

I would like to take a crack at this if you agree.

@griimick Sounds perfect to me!

To start we can just do # of frames, if that's easier for you!