feat: replace simple-git with octokit to load fiddle

[aryanshridhar]

TODOs

• Fix all TODOs in codebase.
• Write tests
• Handle better way to set env variables (probably?)

Fixes #38

[aryanshridhar]

Ready for first round of review @dsanders11 :)

[marfgold1]

I think it's okay if file other than VALID_FILES is being pushed to the contents too. The original Electron Fiddle app allows any other files to be pulled from the gist by prompting if the user wants to add that file first before adding in to the project.