Refactor volume widget

[janfreyberg]

I've made a new widget, VolumeWidget, which adds a bunch of new functionality to the volume widget that takes in nifti files, and misses some of the "custom plotting function" functionality of the NiftiWidget class.

This closes https://github.com/nipy/niwidgets/issues/26, https://github.com/nipy/niwidgets/issues/28, https://github.com/nipy/niwidgets/issues/30

For now I'd propose:

• that this is the new recommended nii-file widget (i.e. the one described in the documentation first)
• that the old functionality that wraps custom nilearn plotting functions stays under NiftiWidget
• down the line, I'd like to re-factor NiftiWidget to look more like VolumeWidget, i.e. a class that implements traitlets / doesn't use widgets.interact. It would probably be renamed to CustomVolumeWidget or something along those lines.
• eventually, I'd like to make all widgets (Surface, Streamlines) look more like this. I think it's a cleaner interface, and I think it will make it easier to be able to export them (eventually...)

To try it, take a look the demo notebook.

[janfreyberg]

@cancan101 - please do share any thoughts on these changes. It would be particularly helpful if you could check if this addresses the issues you've had in the past. To install, simply clone this branch and install the development version with pip install -e repository/path