P0: Allow to import the exported map config

That would also open up the ability to reuse styles across projects or even on multiple datasets in the same project. You could possibly even allow for multiple datasets to be in a single layer with a single style (assuming their data structure is consistent)

1. Save map config via save map choose config only, and downlaod the config json, and store somewhere that can be accessed via url

2. Load kepler.gl/#/demo?mapConfig=http://xxxxx.xxxx.json. Load kepler.gl with a preset config (layers, filters, mapStyles etc).

3.1 The config might contain 1 dataset, in which case kepler.gl try to map layer, filter to it
3.2 The config contain 2 dataset...........

Rules to match config to data: dataset must contain field that layer column and layer color / size by are depends on. must contain fields that filters are depends on....

Currently
if you call `receiveMapconfig` without data preloaded, kepler will try to match layers / filters based on `dataId`, it is not finding those fieds, then it will saved the layer/filter to `layersToBeMerged` `filtersToBeMerged`. When you then call `addDataToMap` without config. kepler will use the to be merged the ones.

For Example:

- If someone saved layer 1 -> dataset a; layer 2 -> dataset b
- upload dataset a -> kepler create layer 1. layer 2 will not be match, it will be stored as `layersToBeMerged` in visState reducer