Add access_token feature for more requests

[ayan-b]

From https://developer.github.com/v3/#rate-limiting:

For unauthenticated requests, the rate limit allows for up to 60 requests per hour. Unauthenticated requests are associated with the originating IP address, and not the user making requests.

A token can be obtained from https://github.com/settings/tokens. localstorage can be used for storing the token for future use: https://developer.mozilla.org/en-US/docs/Web/API/Window/localStorage.