Add labels functionality

[ahuang11]

Couldn't figure out color_index / size_index though because the underlying hv.Labels encounters some issue I've never seen before...

import numpy as np
import holoviews as hv
import pandas as pd
import hvplot.pandas
hv.extension('bokeh')

np.random.seed(9)
data = np.random.rand(10, 2)
text = [chr(65+i) for i in range(10)]
# points = hv.Points(data)
# labels = hv.Labels({('x', 'y'): data, 'text': text}, ['x', 'y'], 'text').options(
#     text_font_size='10pt', xoffset=0.08
# )

# (points.options(color='black', size=5) * labels).redim.range(x=(-0.2, 1.2), y=(-.2, 1.2))

df = pd.DataFrame(data)
df.columns = ['x', 'y']
df['text'] = text
df['color'] = range(len(df))

df.hvplot.labels('x', 'y', 'text', c='color')

hv.Labels(df, ['x', 'y'], ['text', 'color']).options(color_index='color')

[philippjfr]

I'll have a look at the colors issue.

[philippjfr]

The color issue appears to be a bug in holoviews LabelPlot.

[philippjfr]

I'll leave this PR until that is fixed in holoviews.

[philippjfr]

There's now a fix in holoviews so I'll merge soon.