Added append-port flag

Hy spf13!

I've installed hugo on my production system behind nginx (reverse proxy), because I have many servers on the same box.
The automatic URL expanding in the content directory included the port number in the href attributes.
This wasn't good for me because I wanted to serve it normally (:80) on a subdomain, not exposing the "real" port number.

This pull request contains a new flag that doesn't append the port number, when given.