ENH: more options for Zoom plugin

This is an updated version of the project from PR #63 and #112, and adds options to set the pan and zoom limits for the Zoom plugin.  It also adds options for setting the cursor because I don't know which one I like best.

I consider this a work in progress, so feedback is very welcome at this point. Did I mess up the shared axis pan-and-zoom stuff, such as `mpld3/test_plots/test_logscale.py`?