ggplotly doesn't yet implement plot-based title positioning

The latest version of `ggplot2` has a new function that can make the title positioned with regard to the plot rather than the panel. 

`ggplotly` doesn't yet implement this.


```r
library(ggplot2)

plot <- ggplot(mpg) + 
  geom_point(aes(hwy, displ)) + 
  ggtitle("The placement of this title may surprise you") + 
  theme(plot.title.position = "plot")

plotly::ggplotly(plot)
```
