chordNetwork widget in Rcloud notebooks

We have added htmlwidgets support to Rcloud, but unfortunately `chordNetwork` does not work, because it suppresses the viewer, here:
https://github.com/christophergandrud/networkD3/blob/062ed245e596b3dc70f2184e7fcd00a62e917686/R/chordNetwork.R#L129

Maybe the intent was to avoid the RStudio viewer, because it is small, but the current way also avoids other viewers, like the one we defined for Rcloud.

Would you be open to incorporate a simple fix that results the same behaviour in RStudio, but allows other viewers? E.g. only suppressing the viewer if the code is running in RStudio?

The original Rcloud issue for reference: https://github.com/att/rcloud/issues/2169#issuecomment-231205226

I can submit a small PR of course. Thanks!

Start in R/chordNetwork.R around line 129, then inspect how the widget handles viewer suppression in RStudio. Reproduce the chordNetwork behavior in RStudio and Rcloud, and confirm the change preserves the RStudio behavior while allowing the Rcloud viewer to open.

Written by the indexing model from the issue text.