Clean mode accepts input text file

[miketvo]

clean mode should be able to optionally accept a text file containing duplications so that:

• The user doesn't have to wait for duplication detection every time;
• The user can use scan mode first to generate this file, then they can go in and edit it however they like for better control over the cleaning operation.

The format of this file should follow #10 (JSON .imdup file)

[miketvo]

Paths in output file for scan -o/--output will always be absolute, that makes more sense, and easier for clean to parse