add preview port Configuration field

This PR adds a field to the `Configuration` structure called `previewPort` that allows one to specify the default port to listen on when running the preview server. This is useful when a service already runs on the default port of `8000`, since it's a hassle to keep typing `./site preview -p 4000` for example instead of just `./site preview`.

This would be very helpful for me and I imagine/hope others as well. Let me know if you'd like me to change anything!