Support windows

Hi @cliffrowley!

Thanks for starting this atom package. i added some stuff to make this package support opening the GitHub for Windows app with the current repository. It's still a little rough, but the basics work. i changed some code, so i'm sorry to say you may have to test it out to make sure i didn't break the mac stuff... i tried not to!

I saw your comment on tests in the spec folder, I hope to add some tests in the near future. i can do that here or in a separate PR, let me know (if i don't get to it before you can comment).