add support for vertex colors

[trusktr]

we already have the vColor variable in the shader here,

https://github.com/lume/three-meshline/blob/b4a5ebd074b7b3345ee34bb768a9706cfa01e82f/src/THREE.MeshLine.js#L360

we just need to support setting it from a color vertex attribute. Align the API with vertexColors that three.js materials have.