Support factor variables for x and/or y when numeric variable is used to set color

[BeppoBrem]

Support factor variables for x and/or y when numeric variable is used to set color.

See https://github.com/plotly/plotly.R/issues/1883 and also https://stackoverflow.com/questions/45217616/colors-in-plotly-range-not-meaningful-for-factors. The proposed workaround to convert to character before plotting does not honour the order of the factor levels and hence is only a partial solution.

Code can be tested e.g. via: plot_ly(iris, y = ~Sepal.Width, x = ~Species, color = ~Sepal.Length, type = 'scatter', mode = 'markers')

[BeppoBrem]

@cpsievert Can you advise on next steps to get this PR reviewed and eventually merged? Many thanks!