Allow string formatting for labels

[ahuang11]

Closes #1095

I thought about putting this in holoviews, but passing format strings to vdims seems a tad strange...
hv.Labels(kdims=["x", "y"], vdims=["{x} {y}"]), but passing it in hvplot feels natural

import pandas as pd
import hvplot.pandas

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],
    }
)
df.hvplot.points("Longitude", "Latitude", by="City") * df.hvplot.labels(
    "Longitude", "Latitude", "{City} ({Latitude}, {Longitude})", by="City"
)

[maximlt]

This seems like a useful feature to me (even if I have rarely if ever used labels!).

I would have expected to be able to use formatting options like {Density:.2f} except that it raised an error when I tried. We could merge this feature without support for that, it'd be far less useful though. If you try to implement it, I'd suggest leveraging:

• '{a} {b}'.format(a=1) raises a KeyError, so you don't need to check whether some columns are wrongly typed/missing, it does it for you
• '{a} {b}'.format(a=1, b=2, c=3) doesn't raise an error so you can pass all the columns data to .format and it'll happily use only what it needs.

[ahuang11]

I added formatting capabilities.

import pandas as pd
import hvplot.pandas


df = pd.DataFrame({
    "Latitude": [-34.58, -15.78, -33.45],
    "Longitude": [-58.66, -47.91, -70.66],
})
plot = df.hvplot.labels("Longitude", "Latitude", text="{Latitude:.5f} {Longitude:.1f}")
assert plot.data["text"]