Notify the difference from the previous scan result

[knqyf263]

What did you implement:

Closes #255

How did you implement it:

Compare differences from previous results

• Search the old directories if result does not exist in the previous directory

How can we verify it:

$ vuls report -diff

Todos:

You don't have to satisfy all of the following.

• Write tests
• Write documentation
• Check that there aren't other open pull requests for the same issue/feature
• Format your source code by make fmt
• Pass the test by make test
• Provide verification config / commands
• Enable "Allow edits from maintainers" for this PR
• Update the messages below

Is this ready for review?: YES
Is it a breaking change?: NO