Feature: refresh command

[BoboTiG]

What: Proposal for https://github.com/all-contributors/all-contributors/issues/225, it adds the refresh command to refresh the contributors table.

Why:

• As a project maintainer, I want to be able to refresh the README without having to touch the HTML source code.
• As a project maintainer, I want to be able to change the contributors order easily.

How: I first had to split addContributor() to extract pull request code into a specialized function. Then I added the support for the new refresh action, and tests.

Checklist:

• [ ] Documentation (will be done in the specific repository if merged)
• [ ] Ready to be merged
• [ ] Added myself to contributors table. Bot Usage

It's a first draft, hopefully it will be useful. If someone else wants to dig more (like adding better tests), it could be a good base.

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