share, span options for twin x and y axes #205

Description

Hi, I really like the feature that twinx and twiny can be applied to axs, which was not possible with matplotlib.
Would it be possible to add share and span options to twinx and twiny? Such as twinx(sharey=3, spany=True).
Currently, when we specified ylabel for twin axes, it will plot to all axes.

Steps to reproduce

import numpy as np
import proplot as plot

fig, axs = plot.subplots(nrows=2, ncols=2)
axs1 = axs.twinx()

x = np.arange(10)

for ax, ax1 in zip(axs, axs1):
    ax.plot(np.sin(x), c='b')
    ax1.plot(np.cos(x), c='r')
    
axs.format(ycolor='b', ylabel='ylabel1')
axs1.format(ycolor='r', ylabel='ylabel2')

Current behavior:
ylabels and ticks are shown in every plots.

Proplot version

0.6.4