Migrate clap to structopt #14
Description
clap is well-known but heavy and long time not updating, though it says clap team has an upgrading plan this year.
structopt is cooler and lighter than clap, I didn't know this crate when I wrote leetcode-cli otherwise I wouldn't use clap XD.
Well, I find that structopt depends on clap now, not sure if we need this.
reopen if need.