Let user control display/embed behaviour more explicitly

For lack of a better approach, I decided to handle the different ways of displaying/embedding plotly graphs (depending on session mode) with a parameter for the `ggplotly()` method.  This parameter, `session`, can take 3 possible values: `interactive` (default, reminiscent of former implementation), 'knitr' and 'ipynb' (which stands for IPython Notebook, running a native R kernel).
Defaulting to this behaviour has proven useful in the Shiny App use case (where session was not detected as interactive, as expected).
Please review /cc @pedrodz @xsaintmleux @tdhock 
Documenting ipynb use right away.
Good point...  Yes, we may want to call this session mode "notebook".  It's also more readable than "ipynb".  Thanks for the suggestion and for pointing out Jupyter!