[INFRA/EYE] Support to show differences between file's header and license-eye's pattern content

[kwanhur]

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• [X] I had searched in the issues and found no similar feature requirement.

Description

Add subcommand diff in license-eye, it'll help us to show what's the differences between files and license-eye's pattern content, support to specify dir filepath is the better.

Use case

license-eye diff test.go difference example under Apache License2.0

*** asf.lic	        2021-10-25 23:56:27.000000000 +0800
--- test.go	2021-10-25 23:57:04.000000000 +0800
***************
*** 5,9 ****
  // You may obtain a copy of the License at
  //
! //     http://www.apache.org/licenses/LICENSE-2.0
  //
  // Unless required by applicable law or agreed to in writing, software
--- 5,9 ----
  // You may obtain a copy of the License at
  //
! //     wwwhttp://www.apache.org/licenses/LICENSE-2.0
  //
  // Unless required by applicable law or agreed to in writing, software

Related issues

No response

Are you willing to submit a PR?

• [ ] Yes I am willing to submit a PR!

Code of Conduct

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