nimble.ini with only nimbleDir fails
Nothing works with a ``~/.config/nimble.ini`` like:
```
nimbleDir = "/my/home/.config/nimble"
```
where ``/my/home`` should be the user's home directory.  E.g.,
```
nimble install --verbose ANYTHING
    Reading config file at /my/home/.config/nimble/nimble.ini
     Error: Package list '' requires either url or path
```
Providing a ``PackageList`` section pointing to the official repository fixes things, but should not be necessary, IMO.