[Improvement] --pretend option for the convert plugin

Partially resolves #877 showing:
  - Directory creation
  - Copies
  - Deletes
  - Moves
  - Encodings

Information about tagging and plugins on _after_convert_ is not
currently shown. That requires changing the plugins to support the
pretend option, so a lot of work may be needed and it doesn't seem to be
helpful enough for me.