Added display option to Widgets

[jlstevens]

When trying to batch generate notebooks with nbconvert using the execute preprocessor, widgets don't display in the HTML export and show this ugly message:

For this reason, it seems useful to have an option to disable display for notebooks that are to be batch run by setting paramnb.Widgets.display=False. This makes sense to me as widgets don't export to HTML anyway so suppressing those ugly (and irrelevant) warnings seems like a reasonable thing to do.

One other possibility would be to offer a display type. Perhaps a textual display of the current parameter values would be even more useful?

[jlstevens]

In the end setting paramnb.Widgets.display='text' to pretty print the parameter values might be a nicer approach for batch runs...

[jbednar]

Are you sure that the message above won't go away when you run the suggested command? If it's required, it's ok to have a parameter like display, but it would be better to detect that the widgets can't be displayed normally and fall back automatically. It's annoying to have to change the source code in a notebook for something that's not about correctness of the code, but about the capabilities of a particular way of processing the code.