Introduce Option class with Some and None types

Summary:
This change adds an Option class, inspired by Rust generic type Option,
using the same pattern as the Result class in 8b2b9129c24.

First use case is to introduce optional string setting in saas-mediawiki.

Ref T2140

Test Plan: New unit tests to cover fully the Some and None classes.

Maniphest Tasks: T2140

Differential Revision: https://devcentral.nasqueron.org/D3741