chore, never prepend a dash to final filename in explode-by-* mode, ...

use -o foo- or --into abc_ to do the separation

Signed-off-by: Sebastian Freundt <freundt@ga-group.nl>
Merge branch 'cmd/slab' into next

* cmd/slab:
  chore, never prepend a dash to final filename in explode-by-* mode, ...