Add an option to choose color for dataset

**Is your feature request related to a problem? Please describe.**
The problem is that layers at left size panel are colored in color of dataset, which they're using. I didn't find the way to choose dataset color from jupyter keplergl lib. Moreover, you also can't choose color of datasetin your local html copy (when you download your map in html format).

That's quite sad and make some dissonance. I show red and blue points, but at left border they're pink and cyan, people can't understand, which points are which.

**Describe the solution you'd like**
1) An option in lib to choose color for dataset
2) An option in local copy to choose color dataset

**Describe alternatives you've considered**
For now I have to copy my local html in json, feed it to original kepler.gl, manually change the colors of datasets and make a new local html.