Add support for two-factor authentication

[careyrp]

Add support for two factor authentication.

Register a callback function to handle two factor authentication requests.

In the callback, generate a command line prompt for a 2FA key.

[edouard-lopez]

Still got the 2FA error message

env ❯ python2 github-pr-stats --basic my-user https://github.com/org/repo/
Username: my-user
Password:
Enter 2FA code: 366716
Traceback (most recent call last):
  File "github-pr-stats", line 123, in <module>
    url=githubUrl)
  File "github-pr-stats", line 86, in authorize
    return gh.authorize(login, password, scopes, note, note_url, client_id, client_secret)
  File "/home/ed8/.local/lib/python2.7/site-packages/github3/github.py", line 114, in authorize
    json = self._json(self._post(url, data=data), 201)
  File "/home/ed8/.local/lib/python2.7/site-packages/github3/models.py", line 86, in _json
    if self._boolean(response, status_code, 404) and response.content:
  File "/home/ed8/.local/lib/python2.7/site-packages/github3/models.py", line 107, in _boolean
    raise GitHubError(response)
github3.models.GitHubError: 401 Must specify two-factor authentication OTP code.

[careyrp]

The version of github.py is out of date. Need to upgrade to github.py 1.2.0.