core/command: allow seek to "chapter -1"

This will seek to the start of the file regardless of whether the first real
chapter starts there or not.
command: more intuitive chapter seek behavior

If close to chapter start, skipping back goes to previous chapter (no change).
If more than <threshold> seconds in, skipping back will now go to the beginning
of the current chapter instead.

The threshold is set by the new option --chapter-seek-threshold and defaults to
5 seconds.  A negative value disables the new functionality.