Support external link typing

[ghost]

Please add support for external link typing.

https://devdocs.io/html/link_types#external

Indicates that the hyperlink leads to a resource outside the site of the current page; that is, following the link will make the user leave the site.

This allows a link to receive the rel=external attribute so it may be treated by CSS and enable individuals to understand it will take them offsite (important for privacy).

[ghost]

I'd recommend following the pattern used in v1 for blank targeted links so externals may be identified by absolute paths, otherwise it may become necessary to run the parser several times with different options targeted at specific types of links until v3.