properly use non-localhost BaseUrl in server command

Went to give hugo a test drive (looks great btw) and noticed that BaseUrl was being ignored by the server command (e.g., assets/etc would only load from localhost even though any resolving hostname would be served).  For example, I map hostnames/ports to Docker containers, which means localhost won't work as expected - I'm sure this is an edge case, but the flexibility to have it use whatever is passed is nice.

Happy to make any modifications/etc that you'd like, just let me know.