Added argument to options parser indicating on which port to run the web UI

I had a problem running locust on my machine since the port 8089 was already being used by a Django application. While I was able to fix this by changing the source files, I thought it would be much more convenient to be able to specify on which port to run the web server. I set the default port to 8089 but the user can modify this by passing a -P or --port argument. 

Thanks for the good work.