Add a --shout flag that upper-cases the greeting #67

Add pub shout: bool to Options in src/lib.rs, set by --shout in parse_args. In src/main.rs, when set, print the greeting upper-cased (HELLO, ADA!). Add unit tests in src/lib.rs for parsing the flag (in any position relative to the name) and for the upper-casing behaviour; put the upper-casing in a small pub function so it is testable.

Definition of done

• cargo test passes.
• cargo run -- --shout Ada passes.