A custom node for Unity's ShaderGraph to capture lighting and use it into the shader. Works as of July 2018, but the APIs might change!

MainLightNode.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEditor.ShaderGraph;
using System.Reflection;

[Title("Custom", "Main Light")]
public class MainLightNode : CodeFunctionNode
{
	public override bool hasPreview {get {return false;}}

	//This is the string that's passed into the code of the real, final shader code that is used in the Scene when you hit Apply in the ShaderGraph window.
	//As such, it will use real light data from the Scene (for instance, GetMainLight()).
	private static string functionBodyForReals = @"{
			Light mainLight = GetMainLight();
			Color = mainLight.color;
			Direction = mainLight.direction;

			float4 shadowCoord;
			#ifdef _SHADOWS_ENABLED
        	#if SHADOWS_SCREEN
				float4 clipPos = TransformWorldToHClip(WorldPos);
        		shadowCoord = ComputeShadowCoord(clipPos);
        	#else
        		shadowCoord = TransformWorldToShadowCoord(WorldPos);
        	#endif
			mainLight.attenuation = MainLightRealtimeShadowAttenuation(shadowCoord);
        	#endif

			Attenuation = mainLight.attenuation;
		}";
	
	//This string is passed to the node to generate the shader that's used in the ShaderGraph for preview.
	//Since the graph has no conception of what's the main light, we fake the data by hardcoding it in.
	private static string functionBodyPreview = @"{
			Color = 1;
			Direction = float3(-0.5, -.5, 0.5);
			Attenuation = 1;
		}";

	private static bool isPreview;

	//Returns a different string depending on whether we are in the graph or not.
	private static string functionBody
	{
		get
		{
			if(isPreview)
				return functionBodyPreview;
			else
				return functionBodyForReals;
		}
	}

	//Constructor
	public MainLightNode()
	{
		name = "Main Light";
	}
	
	protected override MethodInfo GetFunctionToConvert()
	{
		return GetType().GetMethod("CustomFunction", BindingFlags.Static | BindingFlags.NonPublic);
	}

	//Will calculate the boolean isPreview which is used to decide which of the 2 strings to use (see above)
	public override void GenerateNodeFunction(FunctionRegistry registry, GraphContext graphContext, GenerationMode generationMode)
	{
		isPreview = generationMode == GenerationMode.Preview;

		base.GenerateNodeFunction(registry, graphContext, generationMode);
	}

	//The definition of the ports. 3 go out, 1 goes in
	//(which also has a default binding of WorldSpacePosition, so it doesn't need to be connected)
	private static string CustomFunction(
	[Slot(0, Binding.None)] out Vector3 Direction,
	[Slot(1, Binding.None)] out Vector1 Attenuation,
	[Slot(2, Binding.None)] out Vector3 Color,
	[Slot(3, Binding.WorldSpacePosition)] Vector3 WorldPos)
	{
		//These default values are needed otherwise Unity will complain that the Vector3s are not initialised.
		//They won't be zero in the shader.
		Direction = Vector3.zero;
		Color = Vector3.zero;

		return functionBody;
	}
}