4.8.0 R new features

[bcdunbar]

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

• [x] violin
• [x] table (https://github.com/plotly/documentation/issues/952) refer these examples: https://rpubs.com/bcd/tables
• [x] splom
• [x] scatterpolar
• [x] cone
• [x] streamtube
• [x] MathJax
• [x] 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/)