Add support for custom mesh normals

[Lamoot]

Editing mesh normals is a common approach to adjust the model's shading and is useful for various visual tricks. http://wiki.polycount.com/wiki/Foliage#Vertex_Normals
https://us.v-cdn.net/5021068/uploads/editor/po/l7sdewuycxv7.jpg

Currently the exporter writes default vertex normals but not the "split normals" as they are called in Blender. The FBX and GLTF exporters support this and can be explored to find out the method they used.

[Lamoot]

From my initial research, the custom split normals are found in meshe's loops rather than vertices. I'm exploring other exporters how they do this, but for now it looks there isn't an equivalent of e.g. vertices[0].custom_normals to vertices[0].normals as I hoped there would be.

[Lamoot]

Editing vertex normals manually in the exported. dae file shows custom normals correctly in OpenMW. At least that part works reliably and the only focus is on the exporter.