Avoid panic for unknown flags and invalid selectors when using executable

[kachick]

This PR aims to reduce panic occurrences caused by incorrect user options.
This prevents surprising new users who are not familiar with the tool.

Before

> ./target/debug/scraper --unknown

thread 'main' panicked at scraper/src/main.rs:73:19:
Unrecognized option: 'unknown'
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

> curl -s https://learn.microsoft.com/en-gb/typography/opentype/spec/scripttags | ./target/debug/scraper

thread 'main' panicked at scraper/src/main.rs:116:41:
missing selector
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

After

> ./target/debug/scraper --unknown
Unrecognized option: 'unknown'

> curl -s https://learn.microsoft.com/en-gb/typography/opentype/spec/scripttags | ./target/debug/scraper
missing selector

[kachick]

Thanks for your reviews 🙏