Two-factor authentication support

                   
|Q            |A  |
|---          |---|
|Bug Fix?     |yes|
|New Feature? |yes|
|BC Breaks?   |n  |
|Deprecations?|n  |
|Tests Pass?  |yes|
|Fixed Tickets|   |
|License      |MIT|
|Doc PR       |   |
                   
Adds Two-factor authentication (2fa) support for GitHub.
Closes https://github.com/gushphp/gush/issues/254

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