Dump png's of ggplot2 and Plotly plots, json's and url's

Visual tests are awesome!
We'll be able to check possible diffs on image files and JSON as we re-run the tests (incl. automatically upon pushing new branches).
/cc @pedrodz @chriddyp 
Has this proven necessary?
I know this script is meant for manual use so I'm not too worried.
Just curious.  Thanks, @chriddyp