Feature request: provide optional GitHub organisation authorization support

[aruizca]

Hi @claudioc ,

I already have a full solution for this, but I'd need you to approve my previous PR #105 for me to submit a new one :-)

This PR would add a new optional parameter in GitHub auth config called organisation. If present, the authorization logic in the auth.js file will check if the authenticated user belongs to the required organisation. If not, access is forbidden.

Cheer!