show() should be optional

[eteq]

Continuing in a series of issues that are "confusing elements of the API" from work on #28 :

Right now the visualizations always get shown whenever a glupyter visualization is executed. But there are times when it's useful to create the widget but not show it until it is the last line of a cell. While I can see the merit of either way of doing it, there should be a way to control this so that it's possible to not get the show() on create beahvior

[astrofrog]

@eteq - it's now possible to set show=False when creating a viewer:

https://github.com/glue-viz/glue-jupyter/blob/master/glue_jupyter/app.py#L184

The default is still True though. Are you ok with that?

[astrofrog]

I'll go ahead and close this since I think this has been addressed