ENH: add single-axis zoom

There is something wrong with this, but I hope that pushing it up here a little broken will inspire others to help fix it.  To test:
```
plt.plot([3,1,4,1,5,9,2,6,5,3,5,8], 'ks-', mec='w', ms=10, mew=4)
mpld3.plugins.clear(plt.gcf())
mpld3.plugins.connect(plt.gcf(), mpld3.plugins.Reset(), mpld3.plugins.Zoom(enabled=True))
```

Panning works for me, and double-click zoom works, but pinch touch does not.  I don't have a scroll-wheel to test that right now.  Also, perhaps double-click zoom is not interacting with the panning on the main panel correctly.