Continuous colormaps not working

#548 should have closed this issue, but I still have problems.
If I run the code in issue #327:
```python
import numpy as np
import pandas as pd
import hvplot.pandas

cmap = 'cubehelix'

df = pd.DataFrame(np.random.randn(1000, 10), index=pd.date_range('1/1/2000', periods=1000))
df = df.cumsum()
df.hvplot(colormap=cmap)
```
I actually just get the default colormap, the `colormap` command is completely ignored:

### Software versions:

hvplot 0.7.1
holoviews 1.14.3
bokeh 2.3.1
jupyterlab 3.0.14
Chrome 90.0.4430.93
macOS 10.15.7