c, xoffset and yoffset for labels not working

I'm currently improving the docstrings of `labels`. I can see in `convert.py` that `c`, `xoffset`, `yoffset` should be available as `kind` parameters.


But I cannot see them have any effect.

```python
import hvplot.pandas # noqa
import pandas as pd

df = pd.DataFrame(
{'City': ['Buenos Aires', 'Brasilia', 'Santiago', 'Bogota', 'Caracas'],
'Country': ['Argentina', 'Brazil', 'Chile', 'Colombia', 'Venezuela'],
'Latitude': [-34.58, -15.78, -33.45, 4.60, 10.48],
'Longitude': [-58.66, -47.91, -70.66, -74.08, -66.86],
'Color': ['blue', 'green', 'white', 'black', 'yellow']})

df.hvplot.points(x='Longitude', y='Latitude') * \
df.hvplot.labels(x='Longitude', y='Latitude', text='City', xoffset=100, yoffset=100, c='Color')
```


I would expect the labels not to be drawn on top of the points and to see different colors for the labels.