Ensure WC emails are disabled before generating

[coreymckrill]

Changes proposed in this Pull Request:

This consolidates the steps that each generator class needs to take before generating objects into one initialize method. These steps include creating an instance of the Faker library and unhooking all the callbacks that send transactional emails. This ensures that the emails are disabled regardless of which entrypoint to Smooth Generator is used (cli or web UI).

Fixes #140

How to test the changes in this Pull Request:

1. Install an email catcher plugin like WP Mail Logging so you can see the outgoing emails from your test site.
2. Replicate the issue on trunk first: try generating some products and/or orders, particularly using the web UI rather than wp-cli (WP Admin > Tools > Smooth Generator). Note that emails have been sent about low/no stock when generating some orders, and that emails have been sent about order statuses when generating orders.
3. Now check out this branch and try again. This time there shouldn't be any emails sent.
4. Also try generating things from wp-cli to make sure there aren't any regressions.

Changelog entry

Ensure transactional emails are disabled before generating test content.