Add stroke color generator and config

Adds support for stroke color options. 

Works the same as fill colors (as you can tell by the copy pasta), except you pass in
an array of stroke colors as options. 

Right now this adds `-stroke` onto the end of variable names, but I wonder if this is necessary since we likely won't have an "arrow-left-blue" and "arrow-left-blue-stroke" that are different. 

@tylerball