4.8.0 R new features

[bcdunbar]

New features https://github.com/ropensci/plotly/blob/master/NEWS.md#480 to be documented:

• violin
• table (R table examples #952) refer these examples: https://rpubs.com/bcd/tables
• splom
• scatterpolar
• cone
• streamtube
• MathJax
• Locale
• update animation_button docs

Still to include from 4.7.x

• raster2uri
• geom_dotplot
• coord_ examples

Statistical chart docs from #713

• 2D density (similar to: https://plot.ly/python/density-plots/#2d-histogram-contour-plot-with-histogram-subplots)
• treeplot (w/ igraph) (similar to: https://plot.ly/python/tree-plots/)
• cont error bars (similar to: https://plot.ly/python/continuous-error-bars/)