feat add optional type

[lifeiscontent]

Before submitting a pull request, please make sure the following is done:

• Fork the repository and create your branch from master.
• Run npm install in the repository root.
• If you've fixed a bug or added code that should be tested, add tests!
• Ensure the test suite passes (npm test).

Note. If you find a typo in the documentation, make sure to modify the corresponding source (docs are generated).

[lifeiscontent]

Hey @gcanti let me know if I missed anything here, happy to fix stuff, but as a first time contributor I'm not sure what the protocol is for adding new functionality to io-ts.