Apply parent presence flags when children do not have presence flags

[WesTyler]

Description

When object children do not contain specific presence flags, the value provided to the parent .options({ presence: 'optional||required' }) should be respected for Felicity.example value generation.

Related Issue

Closes #121

Motivation and Context

This allows for bulk flag setting at the parent level instead of requiring the schema to specify .optional() on each and every child schema.

Types of changes

• [x] New feature (non-breaking change which adds functionality. you added at least one new test)

Checklist:

• [x] I have read the CONTRIBUTING document.
• [x] My code follows the Hapi.js style guide.
• [x] I have updated the documentation as needed.
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.