Feature request: colormap with background

Start by inspecting the branca.colormap classes and the generated colormap HTML used by the example. Trace how the colormap is rendered and determine where an optional background rectangle can be represented; done means the existing example supports a selectable background color without changing the color mapping.

Code Sample

import folium
import pandas as pd
import folium.plugins

import branca
import branca.colormap as cm

data = [
    [33.823400, -118.12194, 99.23],
    [33.823500, -118.12294, 95.23],
    [33.823600, -118.12394, 91.23],
    [33.823700, -118.12494, 90.00]
]

df = pd.DataFrame(data, columns=['latitude','longitude','data'])

x_start = (df['latitude'].max() + df['latitude'].min()) / 2
y_start = (df['longitude'].max() + df['longitude'].min()) / 2
start_coord = (x_start, y_start)


colormap = cm.LinearColormap(colors=['red','lightblue'],
                             index=[df['data'].min(),df['data'].max()],
                             vmin=df['data'].min(),vmax=df['data'].max())

colormap.caption = 'Data [units]'

map = folium.Map(location=start_coord, zoom_start=12)


lat = list(df.latitude)
lon = list(df.longitude)
dat = list(df.data)


for loc, p in zip(zip(lat, lon), dat):
    folium.Circle(
        location=loc,
        radius=10,
        fill=True,
        color=colormap(p),
        fill_opacity=0.7
    ).add_to(map)

map.add_child(colormap)
map.save('index.html')

Problem description

Code above creates colored points, and displays a colormap to show what data value each color corresponds to. This is great but it would be even better if a background colour could be selected. This slightly hacky work-around does exactly what I want:
https://stackoverflow.com/a/50427006/4988601

I'd be happy to try and plumb this into folium itself if a few people show some interest in this feature (I'm fairly new to folium so any pointers on what methods/classes I'd need to poke around in would be appreciated).

Expected Output

Colormap with an optional background color rectangle to help it stand out.

Output of folium.__version__

0.10.0