Separate Kaleido from PlotlyJS and introduce AbstractPlot in PlotlyBase

[hhaensel]

I've published a package PlotlySave to implement PlotlyBase together with a saving function independently of PlotlyJS.

This could live side by side with PlotlyJS, but could also be integrated. One way of integration would be the definition of an abstract type AbstractPlot that I just proposed in an issue in PlotlyBase.

I've saved my proposal in respective branches hh-abstractplot of PlotlySave and my forks of PlotlyBase and PlotlyJS.

I'd be happy to hear your opinion either here on in my project's issues.