Add option to disable guid-based item links

[danmactough]

Resolves #293

Summary

• Add a new guidlink option, enabled by default, to preserve current RSS item link inference from HTTP(S) guid values
• Allow callers to disable that fallback so items without explicit links stay linkless
• Cover the behavior with regression tests and update README and TypeScript typings

Testing

• Added unit coverage for both default behavior and guidlink: false
• Updated the typecheck fixture to include the new option
• Full test suite passed locally