Add init for SwiftUI Preview

[zunda-pixel]

struct UserCell_Preview: PreviewProvider {
  static var previews: some View {
    let user = User(
      login: "zunda-pixel",
      avatarURL: "https://avatars.githubusercontent.com/u/47569369?v=4",
      name: "zunda",
      company: "Apple",
      location: "Japan",
      bio: "Hello GitHub",
      createdAt: Date.now
    )

    UserCell(user: user)
  }
}