Allow two-factor authentication in token creation.

* If the user has two-factor auth turned on, we ask for a code
  and use that in creating a token.
* If a token already exists for github-labels but isn't saved,
  we fetch and save it instead of making a new one.
* If the saved token no longer works, we delete it and make a
  new one.
Use library for token creation.

github-oauth-prompt is now being used to create or get a token
for this package.
Merge pull request #3 from henrahmagix/allow-2fa

Allow two-factor authentication in token creation.