Add option for generating code detection summary file

[natiiix]

It would be better than just throwing the output at the user via STDOUT and forcing them to redirect it to a file if they want to access it later without having to re-run the app.

It will probably be just a plain text file in the beginning, but later on it would be nice to generate more sophisticated HTML / PDF (via LaTeX presumably) reports.

Note: The output would still be printed to STDOUT to indicate that the application is still actually running and hasn't gone into an infinite loop or something like that.

Edit: The generated JSON files containing detected clones, mentioned in #69, #70 and #71 should not close this issue, because JSON is not a format that can be easily read by humans.