New feature request: customMapStyle

I would like to request a new feature for the _ReactiveGoogleMap_ component. Currently, it is possible to change the map style by passing a string to the _defaultMapStyle_ prop, for example:
```

```
My use case requires that I provide custom map styles.

Could you add a new prop that can be passed a string, which is a URL pointing to a JSON file containing custom Google map styles? For example:
```
customMapStyle='my/custom/map/styles.json'
```
Thank you.

Start at the ReactiveGoogleMap component and trace how the existing defaultMapStyle prop is handled. Add the requested customMapStyle URL behavior for JSON-based Google map styles, and verify that a map using the example prop applies the loaded custom styles.

Written by the indexing model from the issue text.