A simple context manager for using matplotlib backends

I'm often in the situation where most of the plots in my notebook are fine as static pngs (with `inline`), but it would be nice to interact with some (e.g., change view angle on 3D plots). The `nbagg` backend is nice, but a bit slow for interacting with some figures. I thought it would be nice to have a context manager that allowed code in a given cell to use a different backend.

This is a very simple implementation of something what works on my machine. Let me know if I overlooked something. Will add tests, but first want to make sure this simple approach is ok...

Also, curse my white-space nuker...