add support for basic ipython comm-based message passing

[mathisonian]

This depends on lightning-viz/lightning#26. Adds support for message passing from javascript visualizations to IPython.

This is done through registering handlers on the python side, and emitting comm messages on the javascript side:

javascript

var utils = require('lightning-client-utils');

// in visualization code 
utils.sendCommMessage(this, 'example-event', {myData: 'is an object'})

python

viz = lgn.line(linedata)

def viz_example_event_handler(data):
    # do somethign with data here
    pass

viz.on('example-event', viz_example_event_handler)