Add new map types: Value-by-alpha map

[jgaffuri]

NOTE: can be addressed only after #57.

See for example: http://andywoodruff.com/blog/how-to-make-a-value-by-alpha-map/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3173776/

This requires using 2 statistical values for the map, one for the value (rate/share) and another one for alpha (quantity/weight).

Should be very similar to bivariate-choropleth, except it should use "fill-opacity" style property instead of color combination.