Add support for copying settings from a 'default' repository #29

When managing a project that has a lot of similar repositories, it would be helpful to have one repository with defaults that other repositories inherit from.

How it would work:

1. Create a repository named default, which has templates and a config for new repositories
2. When a new repository is created, it will automatically open a pull request that copies over the templates, one of which would be a .github/config.yml with an option that tells it to inherit from the default
3. Any future changes to the config in the default repository would be synced to repositories that inherit from it