`woocommerce_email` probably not best hook

[crstauf]

In my investigation of #82 and submission of #83 I realized that woocommerce_email hook is loaded on every page, frontend (guest and user) and backend. For sites that have "Fix for WooCommerce emails" enabled (after #83 is merged), that the setting will be applied to all emails.

I'm working on identifying a better hook right now, but this is a problem that should hold #83 until it's resolved.

It is preferable that the action in use:

• is only called when a WooCommerce email is about to be sent
• is an actual action, and not a filter
• is guaranteed to run regardless of customization or source

[crstauf]

From the research I've done so far, there's not really a great action to work with: most of the actions are used to customize the template, and are not guaranteed to be on all emails.

[crstauf]

@MillerMedia Instead of attempting to identify emails from WooCommerce, is it acceptable to adjust the setting to be more generic to HTML emails, and functionally look for Content-Type: text/html in the email headers?

[crstauf]

@MillerMedia ☝️