Allow style functions based upon computed properties

[lucaswoj]

{
  property: ['/', 'population', 'area'],
  stops: [[0, 'red'], [100, 'green']]
}

You can currently style features based on a single property and / or zoom. There are cases in which styling a feature based on a mathematical expression may be desirable, for example drawing a choropleth map of a ratio.

• extend feature-filter to accept mathematical operators
• amend the style spec
• implement the feature in GL JS
• implement the feature in GL Native

[lucaswoj]

Perhaps this syntax could be extended to allow "joining" data with another source:

{
  property: ['getJSON', 'data.json', 'id'],
  stops: [[0, 'red'], [100, 'green']]
}

[jfirebaugh]

Let's keep the discussion in mapbox/mapbox-gl-style-spec#47.