Make options argument to ol.style.Style optional
Make options argument to ol.style.Fill optional
Make options argument to ol.style.Image optional
Make options argument to ol.style.Stroke optional
Make options argument to ol.style.Text optional
Add styleFunction property to ol.Feature
Add ol.layer.Vector.defaultStyleFunction
Use ol.layer.Vector.defaultStyleFunction by default
Don't attempt to render features whose style is undefined or null
Merge pull request #1381 from twpayne/vector-api-feature-style-function

[vector-api] Per-feature styleFunction