Adding Filters to Email Templates

Alright. This is the second attempt at adding filters to email templates. See issue #5692.

I've added an array of styles to `class-wc-emails.php` which are filterable. The styles include all sorts of things. font size, font family, all sorts of colors, email width, etc. These styles are then sent to `email-header.php` & `email-footer.php`.

The only style I didn't add in there was the drop shadow behind the email. Any thoughts on how to do that @jameskoster ? We could just filter a string like `0 0 0 3px rgba(0,0,0,0.025)` but maybe it's better to break it out into multiple items in the array? Ex. `$styles['box_shadow_color'], $styles['box_shadow_blur'], $styles['box_shadow_spread'], & $styles['box_shadow_offset']`