Support notations such as footnotes

[zxdawn]

Description

Sometimes the notations are useful.

Steps to reproduce

import numpy as np
import proplot as pplt
state = np.random.RandomState(51423)
data = 2 * (state.rand(100, 5) - 0.5).cumsum(axis=0)
fig = pplt.figure(suptitle='Single subplot')
ax = fig.subplot(xlabel='x axis', ylabel='y axis')
ax.plot(data, lw=2)
ax.text(-0.1, -0.2, '$^*$Note added to the bottom of chart', transform=ax.transAxes)

Expected behavior: [What you expected to happen] Maybe ax.format(notation='Notes', notation_loc='b') is helpful?