Option for adding errorbar legend

[kinyatoride]

Description

Hi, I think it will be great if we have an option to show error bar in the legend similar to what shade and fade do.

Current behavior

Currently, we cannot show error bar in the legend. Only the line is shown in the left figure.

import numpy as np
import pandas as pd
import proplot as pplt

state = np.random.RandomState(51423)
data = state.rand(20, 8).cumsum(axis=0).cumsum(axis=1)[:, ::-1]
data = data + 20 * state.normal(size=(20, 8)) + 30
data = pd.DataFrame(data, columns=np.arange(0, 16, 2))

fig, axs = pplt.subplots(ncols=2)

ax = axs[0]
h = ax.plot(data, means=True, label='label')
ax.legend(h)

ax = axs[1]
h = ax.plot(data, means=True, shadestd=1, label='label')
ax.legend(h)

Proplot version

3.4.3 0.9.7