GeometryExporter: Changed manual string concatenation with JSON.stringify with prettify. Thanks @sole!
Something tells me that I'll have to refactor (or re-create) SceneExporter...

Same as in DirectionalLightHelper

If light.target.properties.targetInverse is false, no targetLine and
targetSphere is created, we can't update their values on update()!

Removed Vector* add() to addVectors() and addSelf() to add(). Same with sub, cross, min, max, lerp... Renamed Matrix* and Quaternion multiply() to multiplyMatrices(), multiplyQuaternions() and multiplySelf() to multiply(). See #2860.
Good thing we have a ton of examples to make sure nothing is broken. The unit tests also helped. However, I suspect I've left something unchanged.

Added .add(), .addColors(), .addScalar(), .multiply() and .multiplyScalar() to Color. Removed ArrowHelper from DirectionalLight too.

I think it's a bit redundant.

Also, code clean up here and there.