Feature request: Add step back feature (redux) #75

I am conscious this feature has been requested multiple times but everytime it was closed without being addressed properly (for example asciinema/asciinema#278, asciinema/asciinema#133).

I understand stepping back is not as efficient as stepping forward, but couldn't it simply be implemented by replaying the beginning of the file until the desired frame, without timings (like asciinema cat)?

For example, head -50 /path/to/asciinema.recording | asciinema cat -, plays the first 49 frames of the recording. Seeking to previous frame would be as simple as running (the equivalent of) head -49 /path/to/asciinema.recording | asciinema cat -. Seeking to a specific time can be done the same way because we have timing info for each frame.