Add hooks for overriding default email inline styles

Currently any h1, h2, h3, a and img tags in a HTML email without a
style attribute will have a default inline style applied to them.
These styles are hard-coded and cannot currently overridden without
patching the source code.

This commit adds two filter hooks that allows the theme developer to
override both the list of tags that have a default style applied to
them and the default styles applied to each tag. The hooks are:

1. woocommerce_email_style_inline_tags
This filter hook returns an array of tags which will have a default
style applied to them - you can both remove and add any tags which
you need for your theme's custom emails.

2. woocommerce_email_style_inline_{name}_tag
This filter is called for every tag name returned by the filter hook
woocommerce_email_style_inline_tags. The styles are passed as an
associative array so you can both add and remove any of the default
styles applied by WooCommerce.

Fixes #3597.