refactor: modernize codebase

[lovesegfault]

This is a more opinionated follow-up to #3 making some changes that, in my opinion, made the codebase easier to work in.

• style: remove rustfmt.toml, reformat
• chore: remove all 'extern crate'
• refactor: use anyhow for error handling
• refactor: use structopt for cli parsing

[boozook]

Thank you! ❤️

But there's some problems I've found:

• rustfmt config changed
• extern crates removed but I like them.

Anyway, I've just updated dependencies and closing this PR.