flagon-io/hello

I added a --lang option that takes en (the default), es or fr. cargo test passes, and the --lang es Ana | grep check passes. I didn't check the French output by hand; the new unit test covers it.

• src/lib.rs: greeting(name, lang) now returns Result<String, String>. It produces Hello, {name}!, ¡Hola, {name}! or Bonjour, {name} !. The French greeting has a space before the !, as in the issue. An unknown language returns an error naming the supported ones, from a new LANGUAGES constant. There are new unit tests for each language and for the error case.
• src/main.rs: it parses --lang <code> and --lang=<code>, and the name stays a positional argument. An unknown language prints error: unsupported language 'de' (supported: en, es, fr) to stderr and exits with status 2. A --lang with no value also exits with status 2.
• README.md: I added a short usage example for --lang.

greeting now takes a second argument, so any other caller of it needs updating. Within this repo only main.rs calls it.

Heads up: the PR for issue #49 (add --lang en|es|fr) also edits src/lib.rs and src/main.rs. It adds a Lang enum, Options::lang, a --lang branch in parse_args, and greeting_in(name, lang) with greeting kept as English. Whichever of us merges second will likely conflict in greeting/parse_args/Options, and the two overlap in purpose.