Refactor graphql object functions to internal packages

[andyfeller]

As in #13, the extension code is understandably cluttered being a prototype as well as the author's first go project. The spf13/cobra command could be tighter if the logic and structures for GitHub resources was refactored into internal subpackages.

This issue is aimed at some minor refactoring to separate various concerns, leaving the cmd logic cleaner and easier to read.