readme: move to Rust 2018
readme: move to Rust 2018

This updates the README examples to drop the use of `extern crate`. We
do include instructions to add it, but only for Rust 2015.

Closes #552

rust-lang-nursery/regex,rust-lang-nursery/regex,rust-lang/regex,rust-lang/regex,rust-lang/regex,rust-lang-nursery/regex,rust-lang-nursery/regex,rust-lang/regex,rust-lang/regex,rust-lang-nursery/regex