Add color attribute for histograms

[jdugge]

For the use case of using histograms as legends (https://gis.stackexchange.com/q/207208/1931) it is necessary to colour the individual bars of the histogram using different colours.

Plotly currently does not have an option for automatically colouring the bars of a histogram using a colorscale (plotly/plotly.js#1984), but it is possible to provide an array of color names to the marker.color attribute (https://plot.ly/javascript/reference/#histogram-marker-color). If this was exposed using the DataPlotly GUI using a field that allows an expression to be used, it would be possible to create the histogram legend described above.