Add support for shiny showcase mode

[useless5771]

There are quite a few things:

• made sure that additional shiny parameters set via the 'options' list are passed on to appropriate functions
• fixed any issues exposed by enabling showcase mode.
• had to modify functions in shiny that are responsible for displaying extra widgets when showcase mode is enabled. The changes that I had to make are included in the second commit of this PR and they simply expose notebook cells as dashboard source files so shiny can highlight executed lines.

This change relies on this: https://github.com/att/rcloud/pull/2540 .

[gordonwoodhull]

Excellent detective work, @useless5771!

This works beautifully. We will probably never support visual debugging in RCloud, but at least these features demystify Shiny reactive functions nicely.