[dumper] rewrite files by default, add ability to specify whether dumpers should rewrite existing files or not
[demo] ommit unnecessary checks in calculator demo
Merge branch 'dumper-overwrite-existing'

* dumper-overwrite-existing:
  [demo] ommit unnecessary checks in calculator demo
  [dumper] rewrite files by default, add ability to specify whether dumpers should rewrite existing files or not