Add facet_search API functionality

[hmacr]

Pull Request

Related issue

Fixes #503

What does this PR do?

• Add functionality to use the facet-search API
• Add code samples for the new method

PR checklist

Please check if your PR fulfills the following requirements:

• [x] Does this PR fix an existing issue, or have you listed the changes applied in the PR description (and why they are needed)?
• [x] Have you read the contributing guidelines?
• [x] Have you made sure that the title is accurate and descriptive of the changes?

Thank you so much for contributing to Meilisearch!