Adding challenges through external repo fails

Hi,

I may be wrong but it's not possible to add a git repo with `ctf challenge add {repo_url}` at first. This command relies on `git subtree` which is a shell script. this script calls `ensure_clean` to check the HEAD reference but our repo is empty at this point.

I would add a `git commit --allow-empty -m "Project created"` after the `git init` in [__main__.py](https://github.com/CTFd/ctfcli/blob/master/ctfcli/__main__.py).