Add horizontal attribute for colorbars

As of plotly JS v2.8.0 horizontal colorbars are available.

Plotly.R currently uses plotly.js v2.11.1 and the parameter is listed in the R Figure Reference, so it should be possible to make use of this feature.

However I can't find it in schema() and can't apply it:

library(plotly)

fig <- plot_ly(mtcars, x = ~wt, y = ~mpg, color = ~cyl, type = "scatter", mode = "markers")
fig1 <- colorbar(fig, len = 0.5, orientation = "h", bgcolor= "green")
fig1

fig2 <- layout(fig, title = "colorbar orientation", coloraxis = list(colorbar = list(orientation = "h", bgcolor= "green")))
fig2

先从 issue 中的 schema() 和 colorbar()/layout() 示例开始，然后将 R Figure Reference 条目与 Plotly.js v2.11.1 schema 进行比较。跟踪 colorbar 属性的暴露方式，并测试这两个示例；完成的标准是接受 orientation = "h"，并生成水平 colorbar。