Support procuring a user access token from an oauth code

[krishmas]

Support for calling login/oauth/access_token outside of just the DeviceCode flow would be useful for verifying users of GitHub apps that use oauth.

GitHub docs references:

• Warning section of Registering a GitHub App/About the setup URL
• Generating a user access token for a GitHub App/Generating a user access token when a user installs your app

[XAMPPRocky]

Thank you for your issue! I'd be willing to accept a contribution adding this feature.