Node task to generate externs for OpenLayers 3

This is useful for environments where devs want to use a compiled build of ol3 with Closure compiler, but without compiling ol3 together with their app.
I want to compile my application code together with ol3 and I think it would be a shame that we loose types at the API level. Let me see if I can find something better (I don't have good ideas right now I must say).