Add fromNullable constructor in Optional module

[VincentQuillien]

🚀 Feature request

Current Behavior

The fromNullable constructor is missing from the Optional module.

Desired Behavior

import * as L from 'monocle-ts/lib/Lens';
import * as O from 'monocle-ts/lib/Optional';

type Test = {
    a?: { b?: { c: number } };
};

const optionalC = pipe(L.id<Test>(), L.prop('a'), L.fromNullable, O.prop('b'), O.fromNullable, O.prop('c'));
// optionalC: O.Optional<Test, number>