Store Github OAuth token in a seperate file, but refer to that file from the git config

This solves issue #54. This makes it possible for people to keep their `.gitconfig` under version control and share it publicly without having to take care never to leak the access token. 

For now, backwards compatibility with the old config key is maintained: if an OAuth token is set directly, this is used, otherwise it is read from a file. New `git reflow setup` invocations set up the new file-based config.