Change architecture to use charting model until end of pipeline and then draw

[gionkunz]

We should change the architecture of Chartist so that it works with a model of chart elements and wait with drawing until the last step in the pipeline. This would give us several benefits:

• Performance
• More flexible for different rendering adapters (server side, image export etc.)
• Easier to reason about
• Solve some chicken / egg problems (plugins can modify the model before the chart has been drawn)

[titaniumbones]

wondering what the status of this issue is; am mostly interested in using chartist in node as per #780, which is closed but perhaps not implemented yet?

[burzum]

Wanted to use this nice lib as well in an electron app and will have to finde another one because of this blocker. 😢