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.