Add examples for required and repeated options

[Herrtian]

Summary

• add a runnable example for required options and parse errors
• add a runnable example for repeated option values and flags
• link both examples and their commands from the README

Addresses #44.

Testing

• cargo test --all-targets
• cargo test --all-targets --no-default-features
• cargo run --example required_option -- --config config.toml
• cargo run --example repeated_options -- -vv -I src --include vendor