Add some ggplotly() support

[cpsievert]

TODO:

1. Implement ggforce's custom geoms (e.g., GeomArcBar) in a separate PR to ggforce.
2. Implement to_basic.GeomAxisHive (path + text?). For example, see https://github.com/thomasp85/ggraph/blob/8d23845/R/geom_axis_hive.R#L123-L142
3. Add relevant key attributes.
4. Throw warnings/pointers to issues when lack of plotly.js support for color/width arrays is relevant (perhaps this should be done at the plotly level).
5. Catch cases where we need to incorporate geom params into the drawing logic?
6. Tests!

[cpsievert]

Moving to plotly/plotly.R#1326, I think it's probably better to have all the ggforce/ggraph converters in one place that I can quickly change & improve.