NewBehaviourScript.cs
using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class NewBehaviourScript : MonoBehaviour{

	public Material material;
	List<int> triangles     = new List<int>();
	List<Vector3> vertices  = new List<Vector3>();
	List<Vector2> uv        = new List<Vector2>();
	
	void Start(){
		Append(new Voxel(0,0,0,0));
		Init(new GameObject());
	}
	void Append(params Voxel[] voxels){
		foreach(Voxel v in voxels){
			foreach(int t in v.triangles) triangles.Add(vertices.Count+t);
			vertices.AddRange(v.vertices);
			uv.AddRange(v.uv);
		}
	}
	void Init(GameObject g){
		var f = g.AddComponent<MeshFilter>();
		var r = g.AddComponent<MeshRenderer>();
		var m = new Mesh(){
			vertices    = vertices.ToArray(),
			uv          = uv.ToArray(),
			triangles   = triangles.ToArray(),
		};
		f.mesh      = m;
		r.material  = material;
		m.RecalculateNormals();
	}
}

public class Voxel{

	static List<int> data   = new List<int>();
	static int[] order      = {6,7,2,3, 2,3,0,1, 0,1,4,5, 6,2,4,0, 7,6,5,4, 3,7,1,5};
	static int slice        = 8;
	static Voxel(){
		for(int i=0;i<order.Length;i+=4){
			data.AddRange(new []{i,i+1,i+2,i+3,i+2,i+1});
		}
	}
	
	public List<int> triangles      = data;
	public List<Vector3> vertices   = new List<Vector3>();
	public List<Vector2> uv         = new List<Vector2>();
	public Voxel(int x,int y,int z,int type){
		var cube = new Vector3[8];
		var face = new []{0,1,2,1,1,1};
		for(int i=0;i<8;i++){
			cube[i] = new Vector3((i&1)+x,(i>>1&1)+y,(i>>2&1)+z);
		}
		for(int i=0;i<order.Length;i++){
			vertices.Add(cube[order[i]]);
			uv.Add(new Vector2(type+(i&1),slice-(i>>1&1)-face[i>>2])/slice);
		}
	}
}