Expose host in a 'host' source reference

[emilfihlman]

It would be extremely useful to be able to have access to the host section with a source reference such as 'host' or similar. As it stands now access must be explicit which makes writing clean, easy to share configurations harder.

For example, it would be better to be able to write 'self' *.'host' instead of 'self' *.domain.tld to indicate trust to myself and subdomains. This would allow one to build clean configs that are reusable.

Going even further, having . mean the host/origin would be even better, though.