Add rm subcommand to locally remove output artifacts

[huettern]

Implements the feature requested in #3.

• File metadata is checked before removal to catch error if file does not exists (file does not exist error is ignored since it could be that the manifest specifies an output directory, followed by a file inside that directory and thus the files does not exists after the parent directory is removed)
• Outputs specified as directory are removed recursively
• Outputs specified as file are removed normally
• Dry run option with -n/--dry-run prints files to be removed to stdout