Add WC Email send filtering based on object

In my plugin I generate my own emails for subscription orders (order contains a subscription product type)

There does not appear to be anyway using WC Email to accomplish this so this filter is an elegant solution. The is_enabled() method now uses a filter which gets passed the associated email object so the filter can decide to override the email sending.

Using this filter, I can now hook

`woocommerce_email_enabled_customer_completed_order`
`woocommerce_email_enabled_new_order`

examine the order and conditionally disable the email for orders with subscriptions. For regular merchandise I want to use the built in transactional emails.