Svg based heatmaps in Elm.
The general idea is based on simpleheat, which uses a canvas implementation.
The API is inspired by evancz/elm-sortable-table and ohanhi/autoexpand, so do not put your Heatmap.Config into your `Model.
Warning: This is really not the fastest heatmap implementation, mainly because it's rendered in Svg. So it should probably not be used when the heatmap data or config changes a lot on user interaction and when render performance is critical.
Have a look at the /examples/ to get a feeling how heatmaps look, perform and work.