Add support to save and retrieve github tokens locally

[mariocj89]

This needs to be still app specific as you dont want to have multiple apps using the same token

The final objective being the follwing api:

import github_token
token = github_token.get(app_name="my_cool_app", github_api="XXX")

#or 

token = github_token.create(app_name="my_cool_app")

This assumes an interactive app (that can overriden via env vars)

This allows to store all the gothub tokens in a single repository and retrieve them as necessary.
An advante of this is to allow to use github apps with multiple github api endpoints