Work with an existing repo

[AllenDowney]

The current workflow assumes that the user has not created a directory for the project and does not already have a repo for it.

I think I'm not alone in preferring to create the repo on GitHub, clone it, and then run ccds. I would be good to support that.

Or at least provide instructions for how to work with an existing repo.

[pjbull]

I've definitely also hit this.

Do either the --skip-if-file-exists or --overwrite-if-exists flags to the cookiecutter command (which will also work with the ccds command) make that workflow run in the way we would want?