add option to encapsulate

[dwickern]

Checklist

• [ ] run npm run test and npm run benchmark
• [ ] tests and/or benchmarks are included
• [ ] documentation is changed or added
• [ ] commit message and code follows the Developer's Certification of Origin and the Code of conduct

Resolves #197

[dwickern]

Will this change interact safely with fastify-autoload's default with the same name?

[dwickern]

Thanks @Eomm; I pushed your changes to this branch