Add display to code when saving as_svg = TRUE

[olivroy]

Fix #492

Naive fix, (harmless) doesn't change anything ) working on refactoring the render_graph() code to silence some tibble related warnings, but thought I'd send this as a separate PR.

I ran into an error that only shows up when dev (with load_all() when testing this. (that I do not understand)

<std::runtime_error> with text:
  ReferenceError: Viz is not defined
Warnings
In file(con, "r") :
  file("") accepts only open = "w+" and open = "w+b" : using the first one

If you have an idea what it means, please let me know.

But I was able to test it successfully by installing the dev version of DIagrammeR locally.

[rich-iannone]

This seems like an issue with GraphViz (through our viz.js, which incidentally should be updated). Seems like there is an open issue concerning this (but seems benign for us?).

[rich-iannone]

LGTM!