Add support for giving credit to contributors when adding entries

[thanethomson]

When adding changelog entries from the CLI with the add command, we need a way to give credit to contributors. The format we should aim for should be something like:

- This is the changelog entry ([\#123](https://github.com/informalsystems/unclog/issues/123): @user1)
- This is the changelog entry ([\#123](https://github.com/informalsystems/unclog/issues/123): @user1, @user2)