Adds a filter to determine commission on subtotal or total

[JarrydLong]

All Submissions:

• Have you followed the Contributing guidelines?
• Does your code follow the WordPress' coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Adds a filter to queries to determine if we should base commission off of the order subtotal (before tax) or total (after tax).

New filter is called pmpro_affiliates_calculate_on_subtotal and accepts a boolean value

How to test the changes in this Pull Request:

1. Enable the filter add_filter( 'pmpro_affiliates_calculate_on_subtotal', '__return_true' );
2. Run a test checkout with tax in place or edit an order that has earned an affiliate commission - your subtotal and total should be different
3. Commission will be based on the subtotal when enabled

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you successfully run tests with your changes locally?