Ability to specify choropleth map scale colors

Right now, its hardcoded into [choroplethmap.js](https://github.com/ckan/ckanext-mapviews/blob/master/ckanext/mapviews/theme/public/choroplethmap.js). Would be nice if there is some flexibility in specifying the color scale.

Perhaps, a curated selection of certain color palettes that can be specified on a per mapview basis?

Read ckanext/mapviews/theme/public/choroplethmap.js first to locate the hardcoded color scale and understand how mapview settings reach it. Define a supported way for each mapview to choose from curated palettes; done means a mapview can use a selected palette instead of one fixed scale.

Written by the indexing model from the issue text.