Use clap macro for metadata

[ldm0]

Let compiler fetch metadata of clap App from Cargo.toml. :-P

Check: https://docs.rs/clap/2.33.0/clap/macro.crate_version.html

[clearloop]

Coooool!! I have already tired handwriting v0.x.x every time updates leetcode-cli XD...This inspires me that if we can write a macro soft linking readme to lib.rs as well~

Thanks!!!