material.enhancedLightmaps

This is a suggestion that I think could be useful.

`material.lightMap` currently can only darken a material, not lighten it up.
We're using a customised version of Three.js where lightMap is actually working like this:

* value < 0.5: Darken
* value = 0.5: Leave as is
* value > 0.5: Lighten

Together with a few config options (center, falloff and intensity) that is wrapped into the `material.enhancedLightMap` property. If this property is present, it triggers the shaders to work as described above.

Because of the hiding behind the new property it won't break backwards compatibility.

What do you think? For us it's pretty helpful, if we're modelling things like sunlight coming through windows (example http://beta.archilogic.com/JAHT4L)