Configurable Colours

:art:

## Current State
* The colors can be overridden by custom styles for the appropriate selector:
  * Map foreground: ```.hub-map-land```
  * Map background: ```.hub-map-svg```
  * Map water: ```.hub-map-water```

## TODO

* Have a similar configuration schema as seen in [DataMaps](http://datamaps.github.io/)
* Define a range of fill colours with labels. Capable of deriving a legend from.
* Data can be mapped to a fill via ```fillKey```.