text: Title: Add `--global` option to `--install` to save filter config to `global .gitconfig`
  Body: Presently, `nbstripout --install` modifies the repo `.git/config`. This is less portable than saving the path to `nbstripout` and the filters in the user's global `.gitconfig`.
  
  It would be nice to have a command such as:
  `nbstripout --install --global --atributes .gitattributes` that created a `.gitattributes` file in the current repo but saved the filters and path globally. Then, every repo with the `.gitattributes` file would be stripped without needing to install nbstripout in every cloned repository.
  
  See conversation #7 