Use Octkit for Github API

[nkapolcs]

Prerequisite: #25

Use octokit/rest instead of fetch the github api.

[gomorizsolt]

Please make sure it's possible to use the package without authentication.

Authentication is optional for some REST API endpoints accessing public data, but is required for GraphQL queries.

https://octokit.github.io/rest.js/v17#authentication

It's supposed to be used that way according to the docs.

[thisismydesign]

We should prefer no-auth at this point

[gomorizsolt]

We should prefer no-auth at this point

Yep, sure, it was just a side note.