Optionally display remaining time

[vmobilis]

1. Move pretty_progress() calls from sample() to sample_k_diffusion() to account all time taken by sampling cycle.
2. Add pretty_progress() with time remainder.
3. Add configuration option to use that additional function.

Disadvantage: @yslai already implemented DDIM and TCD samplers (#568), and after this change he will need to append show_step(i, steps, &t0); line to end of their cycles.

And thanks for the program!