Add support for setting margins of each navigation items.

[devxoul]

Setting Margins

navigationItem.leftMargin = 0;
navigationItem.rightMargin = 0;

navigationItem.leftMargin = 50;
navigationItem.rightMargin = 20;

System Margins

navigationItem.leftMargin = [UINavigationItem systemMargin]; // 16 on iOS 7+
navigationItem.rightMargin = [UINavigationItem systemMargin];