默认已注册TRC20的合约ABI，底层封装了自动根据合约地址获取合约ABI的机制。通常情况下无需手动注册ABI，仅在某些合约ABI无法自动获取时才需要手动注册。
注册ABI的方式与以太坊一致，例如：
// USDT合约地址：TR7NHqjeKQxGTCi8q8ZY4pL8otSzgjLj6t
let abi = `[{"constant":true,"inputs":[{"name":"who","type":"address"}],"name":"balanceOf","outputs":[{"name":"","type":"uint256"}],"payable":false,"stateMutability":"view","type":"function"}]`
// 注册 balanceOf 方法
exchange.IO("abi", "TR7NHqjeKQxGTCi8q8ZY4pL8otSzgjLj6t", abi)