主要是使用UItabBarItem的attribute keys来设置字体：
Swift 4
UITabBarItem.appearance()
.setTitleTextAttributes(
[NSAttributedStringKey.font: UIFont(name: "Didot", size: 10)!],
for: .normal)
Swift 3
UITabBarItem.appearance()
.setTitleTextAttributes(
[NSFontAttributeName: UIFont(name: "Font-Name", size: 10)!],
for: .normal)