Read repo name from "git config"

When you forked a repository and the issues you're interested in are the upstream's ones, you have to provide repo's name each time you use `ghi`.

I propose the following:

* If `-- user/repo` is provided, use it
* **(the new part)** Else, look for **`git config --local ghi.repo`**, and use it if found
* Finally, fallback to current-user/current-repo