v0.2.3

This is a polish/maintenance release. Changes:

• Update to new versions of regex and scopeguard dependencies, and run cargo update to refresh versions in Cargo.lock.
• Apply cargo clippy fixes.
• Migrate to Rust 2018.

This is also the first crates.io-published version to include the single commit from v0.2.2, which added the -F/--fixed-strings option.