Add possibility to set link MeshMaterial about react-force-graph

[Benkevych]

i need something like this but with gradient color for links from source color to target color.

[vasturiano]

@Benkevych from v1.3.0 I've added a new prop that allows you to specify custom materials for the lines: linkMaterial. It works in a similar fashion as nodeThreeObject. You should be able to add a gradient to the links using material textures, something like:

<ForceGraph3D
  ...
  linkWidth={1}
  linkMaterial={link => new THREE.MeshBasicMaterial({ map: <gradientTexture>, transparent: true })}
>

[Benkevych]

@vasturiano Thanks, works great!