Add better .obj file support

[zakorgy]

• We can emit vertex position and normal after each other, this can spare us one iteration over the vertex data
• Since we only have 6 surface normals, we only have to write it for one cube, and reuse indices
• Only emit surface verticies, this should help on the Renderer side as well
• Investigate possibilities to add color data to the output .obj file

The related code is here: https://github.com/zakorgy/voxel-editor/blob/master/src/editor.rs#L161