Sync all labels using a yaml file

@nedbat 

Assuming you like this approach, I'll update the documentation in the repo (`README`, etc) to be up to date with these changes.
This is a total nitpick, but it's a personal interest of mine, so I'll mention it anyway. `paginated_get` returns a generator, and there's no reason that `get_labels` can't also return a generator -- it's more performant! To do that, just rewrite it like this:

```python
def get_labels(owner_repo):
    url = LABELS_URL.format(owner_repo=owner_repo)
    labels = paginated_get(url)
    for label in labels:
        del label['url']
        yield label
```