add preview port Configuration field

Make it possible to specify the default port to listen on when the
preview server is run. This is useful if another service on the system
already runs on port 8000 (the default), since it's a hassle to keep
providing the port overriding option. For example: ./site preview vs.
./site preview -p 4000
Merge pull request #178 from blaenk/port-option

add preview port Configuration field