Document how to modify layout/styles 

Several issues point out that the current flex style prevents proper control of the content in sidecar, #6, #25.

I have expected `Sidecar(title='', layout={'overflow': 'auto'})` to work as `Sidecar` inherits from `Output` which accepts layout argument (and passes the arguments to HTML), but for Sidecar those appear to be ignored. How do I control layout in the sidecar?