snake_case/camelCase/ClassCase/kebab-case/UNDER_CASE are supported

See https://relay.firefox.com/

sort-with-numbers seems to fail due to missing locales though

- -b uses box-drawing characters
- -H is an alias for --header
- default to stdin instead of requiring file argument
- lint: remove unused import

micro-benchmarked using hyperfine <https://github.com/sharkdp/hyperfine>

in py3, accessing a single byte returns an int, not bytes

in the context, these warnings were pointless but let's fix them anyway